Battery basics - again

[Col_T]

So, I have 5 x 110Ah sealed batteries connected in series e.g. +ve on battery 1 connects to +ve on battery 2, etc, etc, with -ves connected the same way. Connection to the Sterling Inverter Charger is from battery 5, where battery 1 is left-most battery. I know there are risks, but the batteries are left on charge whilst I am away from the boat. Some questions:-

 

1:- Surface charge - if the batteries are left on trickle charge for a long period, think small numbers of days rather than hours, does the surface charge get absorbed into the batteries in the same way that turning off the charger allows the surface charge to be absorbed?

 

2:- I think that if I take a voltage reading across the +ve and -ve terminals of any individual battery I will get a reading for the whole battery bank (because they are all connected) - correct?

 

3:- All the batteries in the bank have two set of posts (one threaded, one plain) and the interconnect cables use the threaded posts. Will a voltage reading, taken from the plain posts, be for that individual battery or would it still be for the whole bank?

 

4:- Am I right in thinking that current flowing from the charger into the batteries = the current drawn by any 12v appliances turned on plus whatever tail current is required by the batteries?

 

5:- I'm thinking of altering the interconnections so that the charger +ve connection goes to battery 4 and the -ve to battery 2, or vice versa - it shouldn't really make a difference - and I think that the cables to the 12v loads should attach to the same points as the battery charger cables. Presumably the +ve 12v load goes to the same battery as the +ve battery charger connection. What would be the impact of connecting the -ve 12v load to the same battery as the +ve battery charger connection - so that both the +ve and -ve on battery 2, for instance, have a total of 3 connections rather than the +ve having 4 connections and the -ve having 2 connections? Alternatively, of course, am I just fretting about nothing?!!

 

That's probably enough to be going on with!!

 

 

Colin T

[WotEver]

-1. That's parallel.

 

1. Yes, but they can get a bit lazy after a while. Nothing that a good discharge & recharge can't fix though.

 

2. Yes, unless your interconnecting leads are too small and you're simultaneously drawing a large current from the bank.

 

3. Whole bank.

 

4. Yes.

 

5. Unless your interconnect cables are particularly small any of those connection methods will be fine.

 

Tony

Edited February 28, 2016 by WotEver

[nicknorman]

Firstly your batteries are wired in parallel, not series! (Fortunately!).

 

1/ yes.

 

2/ yes, unless there are very high currents flowing or very poor connections

 

3/ It makes no difference which post/terminal you use, they are robustly connected together.

 

4/ yes, the charger will supply any boat loads plus whatever the batteries are willing to take, subject to the max current of the charger.

 

5/ there is a view that trying to match the current paths to all batteries is a good idea. It's a view I don't subscribe to PROVIDED the cables are of adequate thickness for the currents involved (ie very low resistance) and all the connections are clean and sound (ie very

low resistance). Personally I would take time to ensure the cables and connections are good and worry less about which terminals things are fitted to. Especially if the boat is routinely in shore power - even if there is a slight mismatch in current distribution all the batteries will eventually be fully charged on shore power.

[WotEver]

 

Yes, you're fretting about nothing

[Col_T]

Thanks for the replies, chaps.

 

I might have been able to say I was starting to get somewhere with this electrickery stuff, if I hadn't got over-confident and used the wrong term for how the batteries are connected together!

 

Another question, if I may. There is a 200A mega-fuse between the battery charger +ve and the battery the charger connects to - should the cables connecting the batteries be 200A rated? I can see why it would be sensible, though it would also seem over-kill to me.

[WotEver]

What size is the charger?

[Col_T]

Sorry, I'm not on the boat at present, but we bought the boat last July and the sales brochure tells me it's a 40 amp charger (Sterling Pro-Combi 1500Q).

 

Hope this helps,

 

 

Colin T

[Paul C]

 

 

 

4:- Am I right in thinking that current flowing from the charger into the batteries = the current drawn by any 12v appliances turned on plus whatever tail current is required by the batteries?

 

 

The way you've worded this is confusing/nonsensical. The current flowing from the charger is NOT the same as the current flowing into the batteries. I think you've got it, but the way you've written the part in red, doesn't read properly. It would be better written:

 

Current from charger = current drawn by any 12V appliances + current into batteries

[WotEver]

Sorry, I'm not on the boat at present, but we bought the boat last July and the sales brochure tells me it's a 40 amp charger (Sterling Pro-Combi 1500Q).

 

Hope this helps,

 

 

Colin T

Sure does

 

If you have a 40A charger then you really only need a 60A fuse in that feed. As you surmised, there's no point having a 200A fuse with cables rated at (for instance) 50A.

 

Your charger cables should be at least 25mm².

 

Tony

 

 

The way you've worded this is confusing/nonsensical. The current flowing from the charger is NOT the same as the current flowing into the batteries. I think you've got it, but the way you've written the part in red, doesn't read properly. It would be better written:

 

Current from charger = current drawn by any 12V appliances + current into batteries

Yeah, but we knew what he meant.

[Loafer]

 

The way you've worded this is confusing/nonsensical. The current flowing from the charger is NOT the same as the current flowing into the batteries. I think you've got it, but the way you've written the part in red, doesn't read properly. It would be better written:

 

Current from charger = current drawn by any 12V appliances + current into batteries

 

I think that was wot Col said, but I prefer your wording.

 

 

Snip:

4:- Am I right in thinking that current flowing from the charger into the batteries = the current drawn by any 12v appliances turned on plus whatever tail current is required by the batteries?

 

End snip

 

 

Correct.

[WotEver]

Correct.

Well, Nick and I both said it was correct too but as Paul points out it is (strictly speaking) incorrect.

 

No, the "current flowing into the batteries" isn't the sum of the battery usage plus appliances. The "current flowing into the batteries" is the current flowing into the batteries.

 

But that's not what Col meant of course.

[Loafer]

Well, Nick and I both said it was correct too but as Paul points out it is (strictly speaking) incorrect.

 

No, the "current flowing into the batteries" isn't the sum of the battery usage plus appliances. The "current flowing into the batteries" is the current flowing into the batteries.

 

But that's not what Col meant of course.

 

But Shirley, if the charger is providing all the current required at that time, it's still all going via the batteries innit? Or does it just rush across the parallel connectors and straight out to the loads?

[Richard10002]

A fuse is there to protect the cable, so a fuse rated higher than the cable is not much use.

 

I'm not saying the cable should be uprated. With a 40A charger you need cable and fuse to cope with the 40A plus a bit...

 

Having said that, if you were thinking of adding an inverter or inverter charger in the future, the 200A fuse might be handy, and the cable might need uprating.

[bozlite]

Sure does

If you have a 40A charger then you really only need a 60A fuse in that feed. As you surmised, there's no point having a 200A fuse with cables rated at (for instance) 50A.

But it's an inverter/charger, isn't it? Pro-combi 1500. So he needs cabling adequate for the 1500W. 200A is a little extravagant, but not hugely so.

[WotEver]

But it's an inverter/charger, isn't it? Pro-combi 1500. So he needs cabling adequate for the 1500W. 200A is a little extravagant, but not hugely so.

Is it? Oh, ok

 

In that case yes, the cable should be rated for >200A. However, if it's big enough for <1V voltage drop then it almost certainly already is

 

Tony

But Shirley, if the charger is providing all the current required at that time, it's still all going via the batteries innit? Or does it just rush across the parallel connectors and straight out to the loads?

No and yes in that order

[Loafer]

Is it? Oh, ok

 

In that case yes, the cable should be rated for >200A. However, if it's big enough for <1V voltage drop then it almost certainly already is

 

Tony

 

No and yes in that order

 

Oh alright then!

[smileypete]

After Mr NMEA pointed this out - I'd prefer to say the fuse protected the circuit, as some items (eg bilge pumps) may rely on the external fuse for fault protection.

 

Also the OP may want to consider buying a clamp meter that can read DC amps, a forum search on clampmeter and "clamp meter" should turn up some examples.

 

It's good practice to arrange the cables in a neat and logical way but without prejudicing performance unduly, makes tracing circuits when fault finding or making modifications a lot easier.

 

cheers, Pete.

~smpt~

Edited February 28, 2016 by smileypete

[Richard10002]

But it's an inverter/charger, isn't it? Pro-combi 1500. So he needs cabling adequate for the 1500W. 200A is a little extravagant, but not hugely so.

Missed that Cable needs to be man enough for the inverter current, and such that the fuse blows before the cable overheats and catches fire.

[Alan Saunders]

But it's an inverter/charger, isn't it? Pro-combi 1500. So he needs cabling adequate for the 1500W. 200A is a little extravagant, but not hugely so.

Sterling Pro Combi Q Specification 12V from: https://cdn.shopify.com/s/files/1/0658/7343/files/Combis_a0837bcc-51d6-47ec-b7b0-7388ad58b76c.pdf?825

Output continuous power: 1600W 2000VA

Surge ratings: 1500W = 4500VA

Low battery trip: 10V

Nominal efficiency : >85%

 

They seem to have swapped the continuous and surge capacity in Watts!

 

I never have understood the distinction between Watts and VoltAmps; maybe it is related to the power factor which is quoted as 0.9% - 1.1%. Using the 2000VA figure a 200A fuse would be correct and the cable would need a minimum rating in excess of this.

 

Maybe the surge capacity does not need accounting for when specifying a fuse; small fuses come in slow, medium and fast blow but I suspect a 200A fuse will be a simple, short length of suitably sized copper. It may be relevant to a contact breaker but Sterling do not specify how long their inverter can supply the surge.

 

So, 1500W at 10V = 150A minimum cable with a larger cross section to reduce voltage drop if the cable is more than a metre (out and back) long. But, nominal efficiency is 85% so, when running the inverter at 1500W it may be drawing 1500/0.85 = 1765W so we need a 177A fuse and the next largest size of fuse is probably 200A.

 

A fuse continuously conducting at its maximum rating will get hot, increasing its resistance and therefore reduce the available voltage at the device.

 

OTOH, you may never draw 1500W from your inverter. Many power tools, vacuum cleaners, even washing machines without water heating, etc. need less than 1000W. e.g. I have an 800W mains (230V AC) drill; if I stall it it will demand 800W+, with no load it consumes perhaps only 50W and if I drill in the lower gear and slowly it may never demand more than 500W.

 

Apologies for the mixture of advice and queries - hopefully, reference to the above Sterling specifications will allow someone with more expertise than I to give a definitive answer.

 

Alan

[nicknorman]

I never have understood the distinction between Watts and VoltAmps; maybe it is related to the power factor which is quoted as 0.9% - 1.1%.

 

Watts is the same as voltamps only if the power factor is 1 - ie voltage and current both exactly in phase. This would be the case for a purely resistive load. However once the power factor strays from 1, either above or below, the voltage and current are no longer in phase and VA will be greater than watts.

[Loafer]

Sterling Pro Combi Q Specification 12V from: https://cdn.shopify.com/s/files/1/0658/7343/files/Combis_a0837bcc-51d6-47ec-b7b0-7388ad58b76c.pdf?825

Output continuous power: 1600W 2000VA

Surge ratings: 1500W = 4500VA

Low battery trip: 10V

Nominal efficiency : >85%

 

They seem to have swapped the continuous and surge capacity in Watts!

 

I never have understood the distinction between Watts and VoltAmps; maybe it is related to the power factor which is quoted as 0.9% - 1.1%. Using the 2000VA figure a 200A fuse would be correct and the cable would need a minimum rating in excess of this.

 

Maybe the surge capacity does not need accounting for when specifying a fuse; small fuses come in slow, medium and fast blow but I suspect a 200A fuse will be a simple, short length of suitably sized copper. It may be relevant to a contact breaker but Sterling do not specify how long their inverter can supply the surge.

 

So, 1500W at 10V = 150A minimum cable with a larger cross section to reduce voltage drop if the cable is more than a metre (out and back) long. But, nominal efficiency is 85% so, when running the inverter at 1500W it may be drawing 1500/0.85 = 1765W so we need a 177A fuse and the next largest size of fuse is probably 200A.

 

A fuse continuously conducting at its maximum rating will get hot, increasing its resistance and therefore reduce the available voltage at the device.

 

OTOH, you may never draw 1500W from your inverter. Many power tools, vacuum cleaners, even washing machines without water heating, etc. need less than 1000W. e.g. I have an 800W mains (230V AC) drill; if I stall it it will demand 800W+, with no load it consumes perhaps only 50W and if I drill in the lower gear and slowly it may never demand more than 500W.

 

Apologies for the mixture of advice and queries - hopefully, reference to the above Sterling specifications will allow someone with more expertise than I to give a definitive answer.

 

Alan

 

What was the question, again?

[MtB]

Watts is the same as voltamps only if the power factor is 1 - ie voltage and current both exactly in phase. This would be the case for a purely resistive load. However once the power factor strays from 1, either above or below, the voltage and current are no longer in phase and VA will be greater than watts.

 

 

How can the power factor possibly rise above 1?

It isn't possible according to my (limited) understanding of PF.

[nicknorman]

How can the power factor possibly rise above 1?It isn't possible according to my (limited) understanding of PF.

Oops, you're right, power factor is the ratio of real power to VA and can therefore only be 0 to 1. What I had in my mind was that the phase angle of the current could lead or lag the voltage but it came out wrong!

[MtB]

Oops, you're right, power factor is the ratio of real power to VA and can therefore only be 0 to 1. What I had in my mind was that the phase angle of the current could lead or lag the voltage but it came out wrong!

 

 

Phew. I thought you might be going to say something that blew my mental model of the universe to smithereens! (Whatever they are...)

I never have understood the distinction between Watts and VoltAmps; maybe it is related to the power factor which is quoted as 0.9% - 1.1%. Using the 2000VA figure a 200A fuse would be correct and the cable would need a minimum rating in excess of this.

 

 

Your thoughts were possibly seduced by this ^^^^^

[nicknorman]

Your thoughts were possibly seduced by this ^^^^^

Yes, I activated fingers before engaging brain.