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Balancing LiFePO4s


MtB

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Well from all my readings on the subject there are two conventional ways to balance one's cells.

 

1) Top balancing, where one charges them one at a time carefully until they are all fully charged, and the voltages are made to all match each by adding little bits of charge above the 'knee' on the lower cells in the set.

 

2) Bottom balancing, where much the same is done when fully discharged, and by further discharging the least discharged a bit at a time until they all exactly match.

 

Both methods are time consuming and tedious, and involve dismantling one's lithium bank and lugging the cells to one's bench power supply. 

 

But I notice a third method now emerging which is an electronic device connected to all 4 cells (or 8 in my case) which balances them automatically with no input from the user and no need to dismantle the bank. Such as at the end of this post.

 

Does anyone know how they work? Or if they work? This one seems to imply (but doesn't actually say) it top balances by limiting the charge voltage as 100% charge is approached. The Valence batteries often mentioned by peterboat also seem to have some self-contained balancing electronics built in, and even the ISDT BG8S battery monitor has a balancing function in the menus, but I can't access the manual to read up on how to use it. (Does anyone here have a PDF of the manual for the BG8S by any chance please?)

 

https://www.ebay.co.uk/itm/10S-36V-30A-BMS-PCB-Protection-Board-18650-Li-ion-Lithium-Battery-Balance-Cell/193196474352?epid=12023254733&hash=item2cfb683ff0:g:gd4AAOSwz2BdwyZj

 

s-l1600.jpg

 

 

 

 

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If you watch more of the videos of the lad you found really irritating, you will see him doing all this kind of stuff, with the same ISDT gizmo you have, and a few BMS’s including one similar to the one you picture :) 

 

As a consequence mostly of his videos, I am pretty much up to speed with this LiFePO4 stuff. Watched quite a few others, but none as readily educational.

 

apropos nothing, I’ve warmed to his manner, appreciate his expertise and, having watched his back story, have a great deal of respect for him.

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Are you using RC batteries as it says it is not suitable for others, these are usually in the 1 to 5 Ah (50000mAh) range fpr model vehicles. It also needs a battery which has the XH 2.54 balance port. So not really for the big batteries we use. The principle used I think is to transfer current from high volt cells to low volt cells automatically until they are balanced. A big Brother version of this would be useful, and is probably what is in the auto balancing 'drop in' batteries that are becoming more available. It does not stop charging on high voltage or low temperatures, that still needs doing externally.  Many of the drop ins also seem to have a max charge or discharge current limit, probably due to internal wire size, whilst charge amps are unlikely to be a problem, discharge when connected to big inverters starting electric motors could easily be.

Edited by Detling
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Weigh both batteries. Place them either side of the fulcrum at distances calculated from their respective weights. Result - Your LiFePO4 batteries are now balanced. You can find cheap Chinese fulcrums on Ebay, but they are liable to catch fire as they are not designed to cope with the extra rolling and pitching they are subjected to when installed on a boat.

 

Jen

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20 minutes ago, Jen-in-Wellies said:

Weigh both batteries. Place them either side of the fulcrum at distances calculated from their respective weights. Result - Your LiFePO4 batteries are now balanced. You can find cheap Chinese fulcrums on Ebay, but they are liable to catch fire as they are not designed to cope with the extra rolling and pitching they are subjected to when installed on a boat.

 

Jen

That chap in the video on the other thread seemed to be balancing them by trimming bits of lead off with a pair of tin snips. Would that work with lithiums, d’you think?

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1 minute ago, BruceinSanity said:

That chap in the video on the other thread seemed to be balancing them by trimming bits of lead off with a pair of tin snips. Would that work with lithiums, d’you think?

Trimming bits of Lithium off with tin snips might. Trimming bits of Lithium off with lead snips perhaps. Trimming bits of Tin off with lead snips possibly. Trimming bits of Lead off with lithium snips could. Trimming bits of Tin off with lithium snips arguably.

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7 minutes ago, Jen-in-Wellies said:

Trimming bits of Lithium off with tin snips might. Trimming bits of Lithium off with lead snips perhaps. Trimming bits of Tin off with lead snips possibly. Trimming bits of Lead off with lithium snips could. Trimming bits of Tin off with lithium snips arguably.

So is that a may be?

 

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50 minutes ago, Detling said:

Are you using RC batteries as it says it is not suitable for others, these are usually in the 1 to 5 Ah (50000mAh) range fpr model vehicles. It also needs a battery which has the XH 2.54 balance port. So not really for the big batteries we use. The principle used I think is to transfer current from high volt cells to low volt cells automatically until they are balanced. A big Brother version of this would be useful, and is probably what is in the auto balancing 'drop in' batteries that are becoming more available.

 

Thanks. I was rather wondering if that was the case. So balancing only happens during charging you reckon? Seems credible. Shame it does not explain  that in the manual! 

 

Back to the drawing board then. I'm sure it must be possible to manually balance the batts in a bank in situ, just need to figure out exactly how.

 

 

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Although my experience is mainly I'm lead acid batteries, i once saw a system using a power transister on each cell to shunt the current as each cell reached its fully charged voltage, thus preventing energy being wasted by overcharging fully charged cells.

 

Should relatively easy to adapt for lithium ion batteries, especially where individual cells are readily accessible.

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3 minutes ago, cuthound said:

Although my experience is mainly I'm lead acid batteries, i once saw a system using a power transister on each cell to shunt the current as each cell reached its fully charged voltage, thus preventing energy being wasted by overcharging fully charged cells.

 

Should relatively easy to adapt for lithium ion batteries, especially where individual cells are readily accessible.

 

Can you suggest which power transistor please, and a detailed circuit design? This is WAY outside my field of expertise! 

 

 

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1 hour ago, Jen-in-Wellies said:

Trimming bits of Lithium off with tin snips might. Trimming bits of Lithium off with lead snips perhaps. Trimming bits of Tin off with lead snips possibly. Trimming bits of Lead off with lithium snips could. Trimming bits of Tin off with lithium snips arguably.

I'm not sure lead snips would hold an edge long enough to cut anything much.

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1 hour ago, cuthound said:

 

Mine too, perhaps @MoominPapa or @nicknorman can help?

My design used MOSFETs, can't remember which offhand The load resistors are 15 ohm, so the shunt current is only around 200mA. I used opto-isolators to drive the MOSFET gates, to make the circuit design easy and avoid level shifters.

 

MP.

 

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The valence batteries stay in balance on discharge as well, its down to the 400 cells per battery I suspect? a little and often perhaps from every cell? In truth I think its down to well made  well matched cells in the first place that makes for a good battery, but an admission has to be made Valence are LiFeMgPo4s so they are different

When I get back from holiday I have 3 x 36 volt Valence batteries that are faulty, I plan to strip them and see if one can supply the parts to repair the other two, I will take pictures and post them on here, they do have BMSs on them so maybe we will understand them better afterwards?

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4 hours ago, Mike the Boilerman said:

 

Well from all my readings on the subject there are two conventional ways to balance one's cells.

 

1) Top balancing, where one charges them one at a time carefully until they are all fully charged, and the voltages are made to all match each by adding little bits of charge above the 'knee' on the lower cells in the set.

 

2) Bottom balancing, where much the same is done when fully discharged, and by further discharging the least discharged a bit at a time until they all exactly match.

 

Both methods are time consuming and tedious, and involve dismantling one's lithium bank and lugging the cells to one's bench power supply. 

 

But I notice a third method now emerging which is an electronic device connected to all 4 cells (or 8 in my case) which balances them automatically with no input from the user and no need to dismantle the bank. Such as at the end of this post.

 

Does anyone know how they work? Or if they work? This one seems to imply (but doesn't actually say) it top balances by limiting the charge voltage as 100% charge is approached. The Valence batteries often mentioned by peterboat also seem to have some self-contained balancing electronics built in, and even the ISDT BG8S battery monitor has a balancing function in the menus, but I can't access the manual to read up on how to use it. (Does anyone here have a PDF of the manual for the BG8S by any chance please?)

https

 

I think the points are that:

 

In order to balance a bank of high capacity cells reasonably expeditiously, you need to be able to pass a lot of current with a voltage drop of 3.5v or whatever, ie quite a lot of power to dissipate. So you need big chunky electronics and heat sinks.

 

Because rapid balancing on the fly is difficult, one can typically either choose to top balance or bottom balance. You can’t achieve both at the same, unless the capacities are identical. If you choose to top balance it means you can fully charge the battery without worrying about a cell over voltage. If you bottom balance it means you can fully discharge the battery without worrying about a cell under-voltage. Take your pick!
 

Obviously this whole thing is a non-issue if you have  well matched cells. If you don’t, it is an issue.

 

The electronic devices you mention, unless they are huge, will achieve the balancing - possibly sometime before you die if you are lucky!

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If you are using an LA as your dump load to protect your alternator then forget bottom balancing as you will be at a voltage where the LAs will be discharging.

If you are installing the cells for the first time the wire all 12 (assuming single cells..... not the multi cell jobbies) in parallel and charge to 3.6V, rest for 12 hours and repeat, then wire to your 3p 4 s config. 

In service every 6 months, I top balanced by making a resistor circuit with 20W wire wound resistors that would take 10A at 3.6V ....I think there are 8 of them ........can't remember the exact number but work it out with ohms law. Work out how much Ahrs you want to take out of a specific cell and wire the circuit up to them. This summer, I reduced one cell by 70Ahrs and another by 50 Ahrs to balance the four. I have now bought one of the modules that Tom (T&B) wrote about to do it automatically. It is quite straight forward once you plot how voltage increases with time when charging. 

You can only balance as you get to the top voltage knee as below that all the voltages will be similar and you will not know which to increase or decrease. On mine, the cell that reaches the knee first is the 3rd highest in the 20-80% full range so you can't balance in this range.

 

 

eta... I use a BG8S to monitor cell voltage and it is designed for radio controlled aeroplanes. It don't work on botes.

Edited by Dr Bob
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24 minutes ago, Dr Bob said:

If you are installing the cells for the first time

 

I'm not, they have been there for a couple of years.

 

25 minutes ago, Dr Bob said:

I top balanced by making a resistor circuit with 20W wire wound resistors that would take 10A at 3.6V

 

This seems a Good Idea. I have 8 cells with spot-welded stainless steel interconnects so no chance of separating them. One cell is significantly lower charged than all the rest, and two cells are significantly higher voltage as 'fully charged' voltage is approached.

 

I'll do the resister thing. Ohm's Law is just the level of complexity I am comfortable with!

 

 

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7 hours ago, Mike the Boilerman said:

 

I'm not, they have been there for a couple of years.

 

 

This seems a Good Idea. I have 8 cells with spot-welded stainless steel interconnects so no chance of separating them. One cell is significantly lower charged than all the rest, and two cells are significantly higher voltage as 'fully charged' voltage is approached.

 

I'll do the resister thing. Ohm's Law is just the level of complexity I am comfortable with!

 

 

When I get myself out of bed I will work out the circuit I made up and take a picture. If one cell is lower than the other two, it would be quicker to just charge up one cell. I have a cheap variable voltage power supply. Which you can set at 3.6V and 7A which can raise one cell. If you read the Norsk site or the marine how to site, one of them talks about charging a 3.3V cell using at 12V charger. Monitor the cell voltage and the Amps going in with a clamp meter and you should be able to put say 20Ahrs in to balance. I have not tried this but maybe someone like Tony or Nick could advise. 

Work out how out of balance the low cell is...i.e. How many Ahrs are needed to put in from your voltage measurements. Let me know if your not sure how to work that out. With my bench supply, I raised my low cell by 6Ahrs in the summer 

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10 minutes ago, Dr Bob said:

I bought a pack of 10 * 1 Ohm 20 W wire wound resistors from RS online. Wire 3 in parallel to give 0.33Ohm and mount on any old piece of metal as a heat sink. When connected across one of the 3.3V cells, it draws about 10A. It does get hot!

Did you need to balance the resistor tolerances first?

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7 minutes ago, rusty69 said:

Did you need to balance the resistor tolerances first?

No that's not required but if you want to be precise, you could get some coarse sandpaper and a set of kitchen scales. Weigh all ten resistors and choose the 3 that are closest. Reject the other 6. Check with middle grandson to check your workings. Carefully sand the heaviest of the 3 on one side only...the one away from the solder connections ...and reweigh every minute until the weight approaches the lowest cell resistor. Keep it away from your knee. It can be painful. Repeat for the next one. Your now have carefully balanced resistors and a sore knee.

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