Parallel batteries, current sharing

wakey_wake · October 31

Replying in a new thread because the source posts are in the monster thread.

44 minutes ago, dmr said:

Don't want to quote your whole posts, but where you have quoted me....

I very much hope that current sharing will be implicit and automatic (as long as the cabling is decent).

The SOC vs voltage curve for lithiums is flatter than for leads, but there is still an obvious slope, so if one battery gets a bit fuller than the other the lower one will just take more charge till things are equal (should have similar cells and BMS in both/all batteries).

TL;DR I don't have data. I do have a growing suspicion that I want to take steps to avoid provoking that into a problem.
Also diagonal (anti-parallel) wiring will equally share current in two identical batteries, provided the lugs are identically crimped i.e. most likely, bought as pre-assembled lengths.

I too had hoped current sharing would be implicit and automatic. I've had most of the parts around for too long and I'm now setting to installing them... and I'm re-thinking. 😞

I still agree that brief imbalance should cause parallel re-balancing when the load comes off.
if heavy inverter load allows one batteries to discharge faster than others, sustaining this can put that battery at a lower voltage
then when the load comes off, the lower battery may be offered >50A recharge current from the group. The BMS should shut down that charge, but then how soon will it come back? How often will it take a recharge overload? What's the risk of doing that repeatedly?
if I got my battery bank into that state, would I know it had happened? What are the symptoms? How would I recover it?

All unknown, and the only way to test it is likely to burn several hundred quid and still not give me good answers. Unless I open the pack and instrument it before abusing it. If I were making youtube vids of it, it might pay back... nah, not for me.

It seems to me that the flatness of the LFP charge curve plateau and the low internal resistance conspire to make this more of an issue that it would be with LA parallel banks. You don't want to blast them with 5C charge, but they'll take it for a moment and nothing bad will happen; also the cell voltages increase to meet you the moment you start charging.

For readers who didn't know, LFP must not be charged below about 0°C. Some Fogstar batteries (at least) have heaters to get around this. More info https://www.perplexity.ai/search/please-explain-briefly-the-min-S24MQonvRNKYeL.Q76TOXw , which says cold charging will shorten battery life and some folks recommend staying above 5°C.

Previously I had worried that a battery in the group could get trapped in a differently-charged state by cold,

they're all cold-ish and empty-ish, but some are warm enough to allow charge
then you accidentally split the SoC by charging some
what happens next? I don't want to find out the hard way.
To avoid it: batteries closer to the baseplate are better for roll stability, and better for staying ~4°C over winter.
Keeping temperature history for the tops and bottoms of the batteries should allow inference about possible temperatures inside. This should be enough to be sufficiently cautious about charging in a near-cold environment.
If I want heater pads later, I can add them. Not as good as having them inside the battery between cells, but surely cheaper.
If I had a shunt on each battery, instead of one on the whole bank, I could know what's going on all the time.

Shunt and/or fuse on each battery is barely compatible with the diagonal (anti-parallel) bank wiring scheme anyway.

44 minutes ago, dmr said:

I am charging from an alternator controlled by an Arco Zeus, plus a Victron DC-DC and a Victron Solar. I have set suitable charge voltages so that the Victron stuff stops early and the Zeus does the last bit of charge to float. It all works fine and goes into float correctly. The BMS is thus my second line of defence for emergencies only. I will have individual MRBF fuses on both batteries. I have not installed the DIY battery yet (have just had a huge diversion) but will take it to overvoltage and undervoltage first to confirm that the BMS really works.

and as you almost said "you can't protect a semiconductor with a fuse" 😀

Found https://arcomarine.com/products/arco-zeus-high-energy-alternator-regulator

(I can't guess why Bluetooth is considered a feature not a liability, but it doesn't really matter. I'm not in their target market.)

Also earlier,

On 07/08/2025 at 10:50, dmr said:

I feel the same, especially as I am just about to install my new DIY jobbie next to the big Fogstar, and like you I sleep next to them.. I am going to keep a closer eye on battery temperature. But then again I have seen a lead acid explode and thats probably worse as you get zero warning.

2 hours ago, wakey_wake said:

I'm interested in how you plan to manage even current sharing among a population of batteries.

@dmr You're using the DC-DC to share charge with the engine starter? Both ways?

You're charging the domestic directly from the alternator? What about the risk of load dump when the BMS freaks on you without giving notice to the Arco Zeus? In this setup I would imagine giving the Arco the low threshold (to reduce the alternator output early) and let the DC-DC top them off.

You said "both batteries" which sounds like your domestic bank is only one LFP unit, of maybe 100Ah or 200Ah? Ah no, moving from domestic bank of one to two?

I'm using "battery bank" or "battery group" to mean several consumer-packaged 12.8V 100Ah LFP batteries of 4 cells each, which I will parallel one way or another. There are 5x 12.8V 100Ah for the domestic bank I want balanced.

The alternator will charge the engine battery as usual. I'll charge the domestic from that, by VSR and/or DC-DC depending on how it works out. This is my way to avoid the risk of load dumps, because the engine starter will never disconnect while the engine runs. I am less certain that shedding 150A of charge current won't cause a high transient voltage (over microseconds?) before the LA battery chemistry catches up... but the current alternator struggles to give 40A anyway. Problem for another day.

What I had been doing, before the distraction of the 50-page thread, is writing Python to calculate current sharing (iteratively) in the 5-way diagonal scheme. I also wanted to know the effect of putting a current-clamp and/or fuse leg on each negative. This is incomplete, but early results (if not bugs) are telling me that diagonal wiring is no silver bullet and the larger the group the worse it gets.

Consequently I'm already moving my thoughts to busbar or star connection, with equal length cable to each battery.

wakey_wake · October 31

On load dumps,

On 09/08/2025 at 08:34, dmr said:

(I made myself an alternator discinnect protector)

which covers that! Another interesting thread at

dmr · October 31

I think you just might be over thinking all this.

I assume the two batteries will equalise their states of charge during normal charging and discharging. The terminal voltage will be a (sort of) mean of the two batteries so if the batteries get out of step one will take more current, the other will take less.

I am assuming and hoping that I never see a BMS disconnect.

My batteries are indoors in the engine room) (which is a bit scary), but as a result I suspect they will never get below about 5 degC so cold charging and heating are not an issue. If I ever leave the boat during a real cold spell then I will have to manually sort things out, probably by setting a very low charge current.

I do not use large amounts of electricity so want batteries that can deliver a few amps for a long time, rather than lots of amps short term, and my charging is not excessive, so even if one battery did disconnect the other would not see out of spec currents.

The batteries charge from the domestic alternator controlled by the Zeus, the Victron Orion then supplements this from the starter alternator. Im still optimising things but the Victron shuts down at about 14 volts leaving the Zeus to do the last bit of charging up to 14.2 volts and then floating. I could have done this the other way round but find the Zeus a bit easier to adjust than the Victron. I still need to do an option to stop charging at 90% using the BMV relay.

I have done very little boating this year due to lack of water on the Rochdale, so engine running is mostly just to charge the batteries, I just stop when they are up at 80% or 90%. Ive had a couple of huge distractions of late including accidentally getting a full time job (short term) but will get back on boat projects very soon.

wakey_wake · October 31

21 minutes ago, dmr said:

I think you just might be over thinking all this.

You're not the first person to say this. 🙄

21 minutes ago, dmr said:

I assume the two batteries will equalise their states of charge during normal charging and discharging. The terminal voltage will be a (sort of) mean of the two batteries so if the batteries get out of step one will take more current, the other will take less.

[...]

I do not use large amounts of electricity so want batteries that can deliver a few amps for a long time, rather than lots of amps short term, and my charging is not excessive, so even if one battery did disconnect the other would not see out of spec currents.

For two batteries in diagonal / anti-parallel, yes. Also for a tree of such batteries, each in diagonal parallel.

My bank is wider, and no it's not going to fair-share. If it catches up after then it's still good but I can't calculate that.

99% of the time the peak load will be the kettle ☕ and the major drain is refrigeration, but I don't want the 1% oddity to trash my system or worse.

Reading of the ABC incident (with much info missing) knocked me back from "LFPs are pretty safe, it'll be fine".

@nicknorman's comment (other thread) is really helpful though. I had not thought to look at this and it will assist recharge/imbalance recovery across the plateau.

On 09/08/2025 at 12:21, nicknorman said:

Although Li has a low impedance, there is still some “hysteresis” between discharging and charging. Ie if you add 50mV to a rested battery, you don’t get much current.

(Hmm, also might be relevant to the odd results I got when attempting to use what might be a mixed-metal-oxides coated piece of expmet as a reference electrode, because I'm unwilling to spring for platinum just to mess about. It didn't do what I expected, and it seemed to show hysteresis. https://www.ebay.co.uk/itm/223756561497 )

David Mack · November 1

5 hours ago, wakey_wake said:

I'm using "battery bank" or "battery group" to mean several consumer-packaged 12.8V 100Ah LFP batteries of 4 cells each, which I will parallel one way or another. There are 5x 12.8V 100Ah for the domestic bank I want balanced.

With Fogstar selling single batteries up to 628Ah why do you need to parallel batteries in the first place?

MtB · November 1

8 hours ago, David Mack said:

With Fogstar selling single batteries up to 628Ah why do you need to parallel batteries in the first place?

Have you tried picking up a 628AH LFP battery?!

Alan de Enfield · November 1

7 minutes ago, MtB said:

Have you tried picking up a 628AH LFP battery?!

60 kgs - a little heavier than my 56kg 230Ah FLAs. and, I have 6 of them.

(The AGM 230Ah version weighs 67kgs)

Edited November 1 by Alan de Enfield

Sailbadthesinner · November 1

Wakey, if as you suggested you were to put shunt on each individual battery to measure current and Ah I am certain you would find thst you have nothing to worry about.

You are indeed over thinking this.

dmr · November 1

The big Fogstar Lithiums are heavy, more than I would want to lift, but within the capabilities of a strong young man, I got one to help me but due to limited space he said it was easier to do it my himself.

The BMS will have a built in shunt and current measurement, not nearly as good as a proper Victron shunt, but good enough to check that charging currents are roughly in balance.

As a full time off grid liveaboard I want two batteries for redundancy, I don'y want all my creature comforts to be dependant on a Chinese BMS that is not always quick and easy to replace.

I would not worry about diagonal connection stuff, just use busbars, but do note that some of the chinese items will likely struggle to meet their current spec. so get well over-rated ones, or enhance them with a nice thick copper bar.

David Mack · November 1

5 hours ago, MtB said:

Have you tried picking up a 628AH LFP battery?!

The Fogstar Drift 628Ah weighs 49Kg, but the Drift Pro has a bigger case and weighs over 60Kg. Both use the same cells. So what else in the Pro results in so much greater weight and size? Surely not just the CAN port and Bluetooth module.

MtB · November 1

23 minutes ago, David Mack said:

The Fogstar Drift 628Ah weighs 49Kg, but the Drift Pro has a bigger case and weighs over 60Kg. Both use the same cells. So what else in the Pro results in so much greater weight and size? Surely not just the CAN port and Bluetooth module.

I find manhandling standard 25kg leisure batteries in and out of the awkward places they get installed in boats difficult enough let alone a battery weighing 49kg.

And yes its hard to imagine what on earth weighs the extra 11kg inside the Pro if it isn't the cells. Ballast perhaps, so it floats the right way up if you drop it in the cut?! 🤣

Edited November 1 by MtB
add a missing bit

Phoenix_V · November 1

1 hour ago, David Mack said:

The Fogstar Drift 628Ah weighs 49Kg, but the Drift Pro has a bigger case and weighs over 60Kg. Both use the same cells. So what else in the Pro results in so much greater weight and size? Surely not just the CAN port and Bluetooth module.

Metal case and cage in the latter

wakey_wake · November 1

6 hours ago, Sailbadthesinner said:

Wakey, if as you suggested you were to put shunt on each individual battery to measure current and Ah I am certain you would find thst you have nothing to worry about.

You are indeed over thinking this.

Of course I am. Still, would rather overthink it several times than shell out for expensive kit and think again afterwards.

It seems increasingly sensible to put a fuse or breaker on each battery in the parallel bank.

I'll use the neg leg because less risk of live metal touching the hull.

Then I can (ab)use a shunt monitor to get an approximation of the current and also to detect fuse blow.

16 hours ago, David Mack said:

With Fogstar selling single batteries up to 628Ah why do you need to parallel batteries in the first place?

Can't afford it, can't lift it, can't fit it in my boat, prefer to have some physical gaps between that much energy storage. I am not in their target market.

dmr · November 1

The mrbf "terminal" fuses are the way to go, though pay extra and get a proper (Bluesea) version and fit with care as the contact area is quite low. These go right on the battery terminal so protect against any wiring accidents. Fuses go on the POSITIVE terminal.

Gybe Ho · November 1

38 minutes ago, wakey_wake said:

It seems increasingly sensible to put a fuse or breaker on each battery in the parallel bank.

The Victron Power-In class-T is designed for this very purpose. A JBD BMS has a pin-out for a logical switch that will power off the BMS and thus disconnect the battery cells.

38 minutes ago, wakey_wake said:

Then I can (ab)use a shunt monitor to get an approximation of the current and also to detect fuse blow.

Your ears and nose will let you know, a class T cell blowing is a spectacular event.

Funny and educational video.

https://www.youtube.com/watch?v=WL02t1aW6Fc

Edited November 1 by Gybe Ho

wakey_wake · November 1

Ah sorry to be double fussy but..

18 minutes ago, dmr said:

The mrbf "terminal" fuses are the way to go, though pay extra and get a proper (Bluesea) version and fit with care as the contact area is quite low. These go right on the battery terminal so protect against any wiring accidents.

I like the idea of the MRBF, leaving no place a sneaky spanner can get upstream of it. I suspect they might suit me after the patent wears off (I have not looked to see if they're still in patent).

The price puts me off, I suspect they won't fit some of my batteries (no terminal post, only an M8 bolt), and if I need a spare in a hurry which shop will I go to?

I'm happy with my Mega-style fuses. BTW if you decide to test-pop a fuse, the short circuit is a generous test because all fuses will pop that way. Getting hold of a controlled over-current is a bit harder.

18 minutes ago, dmr said:

Fuses go on the POSITIVE terminal.

There are good reasons to do that, yes. I guess if it's an MRBF you have to, else it won't fit.

However I think I have a better reason to put the additional fuse in the negative leg of each battery - the install will still comply with BSS because I'll have the isolator and positive fuses also.

The only reason it would be problem for me is a technical reason or if some bureaucracy forbids fuses in the negative side. The only technical reason I can think of is the possibility of a battery-side short of fuse to hull, and that is a much scarier prospect in the positive leg.

David Mack · November 1

1 hour ago, wakey_wake said:

Can't afford it, can't lift it, can't fit it in my boat, prefer to have some physical gaps between that much energy storage. I am not in their target market.

Price per Ah is cheaper with the larger batteries. And that's without the extra cost of wiring and terminals for the interconnects.

jonathanA · November 1

28 minutes ago, David Mack said:

Price per Ah is cheaper with the larger batteries. And that's without the extra cost of wiring and terminals for the interconnects.

All true of course BUT most of the bigger packaged frog spawn batteries have a 250A BMS so you might find your 500 or 600AH drift can't run your 3 or 5 kW inverter....

which might just be a reason parallel a couple. Although God only knows why you would want to fit fuses and shunts per battery seems massive overkill to me... not my money so good luck....

dmr · November 1

7 minutes ago, jonathanA said:

All true of course BUT most of the bigger packaged frog spawn batteries have a 250A BMS so you might find your 500 or 600AH drift can't run your 3 or 5 kW inverter....

which might just be a reason parallel a couple. Although God only knows why you would want to fit fuses and shunts per battery seems massive overkill to me... not my money so good luck....

Fogstar stuff looks quite good to me but I have seen a couple of reports of terminals running hot, and the BMS jobbies are from China so I would apply my Chinese rule and try not to run anything above about 50% of its spec, so thats 125 amps for a 250amp BMS. I have the 300amp BMS and intuitively I would not want to put 300amps through it. Two batteries in parallel makes a lot of sense to me.

Gybe Ho · November 1

14 minutes ago, jonathanA said:

All true of course BUT most of the bigger packaged frog spawn batteries have a 250A BMS so you might find your 500 or 600AH drift can't run your 3 or 5 kW inverter....

They use a 300A BMS in some of their big jobbies, I have two such BMS boards supplied by Fogstar. Like some here I opted for two batteries, in theory my system is good for 600A and is fused for 400A. A 3K invertor needs around 220A with some headroom for transient peak loads under 305A.

I have been reading up on options to introduce a new battery to an existing bank. Some BMS boards support a max current limiter that is separate to over current cut off. The intension here is to allow two batteries at different states of charge to rebalance against each other without a damaging rush of current between them.

Gybe Ho · November 1

1 hour ago, wakey_wake said:

I like the idea of the MRBF, leaving no place a sneaky spanner can get upstream of it... I'm happy with my Mega-style fuses.

Will your insurance company agree with that design decision while picking over the charred remains of your boat? Class T fuses address the specific problems of trying to fuse lithium batteries.

PeterF · November 1

I have a pair of 12V 300Ah Victron NG lithium batteries which do not have an internal BMS but do have some monitoring including a shunt in each plus the shunt in the external Lynx NG BMS. The batteries are connected direct to the BMS lugs with 2 identical length cables on + and - with 300A Class T fuses. I charge at 150A max and discharge is typically 160A max. I see little imbalance in the current flow between the 2 batteries, but as charging approaches absorption voltage and during the short absorption period I do see a bit of current difference, maybe 5% difference as any imbalance is corrected. They do not report individual SOC.

The trouble with a lot of the cheap drop in batteries is the SOC calculated by the BMS can be wildly out, if they are reporting different SOC you do not know if it is real or not. Also, with a lot of these batteries, the wiring gauge inside the case is much smaller that used outside the case, hence any internal wiring discrepancies from battery to battery are likely to be more pronounced.

wakey_wake · November 1

46 minutes ago, dmr said:

[...] I would apply my Chinese rule and try not to run anything above about 50% of its spec, so thats 125 amps for a 250amp BMS.

Not to disagree, but to generalise: pushing anything to 100% of its spec is asking for trouble.

What I suspect happens in cheaper brands is, components are pushed much closer to spec and there is insufficient margin for variability or error.

Another aspect of this is that if the specifier and buyer of a part (e.g. BMS board) does not reverse engineer it right back to components and check all the specifications, they can't tell where the corners have been cut too close. And who's going to go to that much trouble when buying in a part?

So it compounds. Your rule to run the part to 50% of its advertised (potentially over-optimistic) spec is a valid workaround for some of the mess upstream. 😞

1 hour ago, jonathanA said:

Although God only knows why you would want to fit fuses and shunts per battery seems massive overkill to me... not my money so good luck....

I would rather buy some extra fuse holders and fuses, than risk myself and the boat.

Beyond that, I would rather understand the risks enough to take the sensible precautions and ignore the irrelevant ones. What's sensible? Well the ABC explosion suggests that "industry best practices" may not always be the whole answer. Hence I'm back to overthinking.

I briefly considered per-battery shunts - because I want to know what's going on between them, because I suspect what could go on between them might escalate into serious safety problems. However there is no active protection in this.

Fuses per battery are (bizarrely) more expensive than shunts per battery, and have a potential running cost. However they have a chance of being effective in the face of a BMS failing short or catastrophic overcharge into [ one shorted cell, three imperilled cells ] from the rest of the bank.

What I'm then guessing (untested) is I can use the voltage drop across the fuse as a shunt-like input.

If I'm making up a PCB for two INA238 then it might as well have space for eight, because I'll have to buy a tube of 10 anyway.

~~~~~ merge?

15 minutes ago, PeterF said:

The trouble with a lot of the cheap drop in batteries is the SOC calculated by the BMS can be wildly out, if they are reporting different SOC you do not know if it is real or not.

I'm not specifically planning any SoC. We'll see how that works out.

I know I can charge count the shunt, but the power cost of watching it continuously might be a chunk of the whole budget on a quiet winter day.

15 minutes ago, PeterF said:

Also, with a lot of these batteries, the wiring gauge inside the case is much smaller that used outside the case, hence any internal wiring discrepancies from battery to battery are likely to be more pronounced.

Fair point which I had not considered. However it's the longer runs that are going to eat my volts.

My physical jigsaw puzzle is giving me unwelcome options

cram them in aggressively, with less space between than I'm comfortable with. 🔥😨
space them further and buy more copper to cross the gaps, 💸
however this combines better with the star / busbar style.
reduce to 4x 12V and make the last one my 24V topper. 😞
put them all on the uxter plate, making unwelcome clutter, higher centre of gravity, possibly worse thermal influence of the lower water. 😞

24V topper? I want 24V available for some kit, without having to run a 12->24V boost continuously. Sticking one of the tired old LA domestics on top of the 12V LFP bank seems like the way, and it can be charged from the 12V bank as needed.

dmr · November 1

19 minutes ago, wakey_wake said:

Not to disagree, but to generalise: pushing anything to 100% of its spec is asking for trouble.

What I suspect happens in cheaper brands is, components are pushed much closer to spec and there is insufficient margin for variability or error.

Another aspect of this is that if the specifier and buyer of a part (e.g. BMS board) does not reverse engineer it right back to components and check all the specifications, they can't tell where the corners have been cut too close. And who's going to go to that much trouble when buying in a part?

So it compounds. Your rule to run the part to 50% of its advertised (potentially over-optimistic) spec is a valid workaround for some of the mess upstream. 😞

I would rather buy some extra fuse holders and fuses, than risk myself and the boat.

Beyond that, I would rather understand the risks enough to take the sensible precautions and ignore the irrelevant ones. What's sensible? Well the ABC explosion suggests that "industry best practices" may not always be the whole answer. Hence I'm back to overthinking.

I briefly considered per-battery shunts - because I want to know what's going on between them, because I suspect what could go on between them might escalate into serious safety problems. However there is no active protection in this.

Fuses per battery are (bizarrely) more expensive than shunts per battery, and have a potential running cost. However they have a chance of being effective in the face of a BMS failing short or catastrophic overcharge into [ one shorted cell, three imperilled cells ] from the rest of the bank.

What I'm then guessing (untested) is I can use the voltage drop across the fuse as a shunt-like input.

If I'm making up a PCB for two INA238 then it might as well have space for eight, because I'll have to buy a tube of 10 anyway.

Im sure a Bluesea mrbf fuse holder and a couple of spare fuses works out a lot cheaper than a Victron smartshunt or similar.

The 300amp BMS as used in the Fogstar Pro has busbars soldered to the PCB but they are really not very chunky at all. Each has a m6 bolt to take the cable connection. It does not really feel like 300amp engineering, but I guess that as long as everything is spot on then 100amps per section might just be ok, but if any bad crimping or whatever causes a current imbalance I would worry.

and yes, you could use a fuse as a cheap shunt but I would not expect good results, but you certainly can measure a few millivolts accross them.

I like the Chinese 50% rule but can't claim credit, I think it came from Sunshine Solar (or similar company) when I talked to them about their unbranded inverters.

wakey_wake · November 1

43 minutes ago, dmr said:

Im sure a Bluesea mrbf fuse holder and a couple of spare fuses works out a lot cheaper than a Victron smartshunt or similar.

I got chunky metal shunts and will use INA238 (I²C shunt monitor) chips. They can do low-side monitoring and handle up to 85V.

Anticipating the reply "Some people have too much time" etc. in making my own - it's just part of the fun. 😁

43 minutes ago, dmr said:

[...] you could use a fuse as a cheap shunt but I would not expect good results, but you certainly can measure a few millivolts accross them.

That's all it needs, I'm not expecting precision. What I want from fuse-as-shunt is "is this battery roughly in line with its siblings?" so it's possible that a mess of opamps is a better tool than the INA238... it will become clearer when I get closer to it.

Now I think of it, at times I've used resident 25mm² cable as a shunt-substitute. It doesn't drop many mV but the meter can see them.

I have a 500A shunt for the bank and will probably add a much smaller shunt on the non-inverter domestic distribution.

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