What would you do first? Bottom or top balancing?
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Ampster
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First? It is one or the other, not both. They defeat each other so there is no reason to do both. If I bottom balanced a pack I would have to wait weeks as the BMS top balanced the pack at 20 mAmps. I would not build a pack without a BMS.
For a while there was a cult like following in the DIY EV community for bottom balancing and NOT using a BMS.
For a while there was a cult like following in the DIY EV community for bottom balancing and NOT using a BMS.
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Top balance and call it a day. Ideally one cell at a time, but I know this is not practical for many. Instead, hook them up as you normally would for th pack, add BMS, charge until BMS cuts off a cell (put high voltage disconnect at 3.6 or below per cell). Dismantle, put cells in parallel, hook up a CV/CC power supply and set to 3.6 (before connecting!!), then connect to the parallel cells to balance them, but make sure you don't use a low current over a long period of time. You can overcharge a cell by keeping it at 3.6 (even below that) for a long time at low C.
This might help you visualize why you can't top and bottom balance:
The balancing process is basically taking slightly dissimilar capacity cells and 'balancing them' at either the top or the bottom so they either reach full or reach empty at the same time, but you cannot achieve both unless they were theoretically perfectly matched in which case only one would be necessary to achieve both. At least that is my understanding
Bottom balancing was supposedly popularized with EV's, but doesn't make much sense outside of that application and a select few others where (1) high C rates are normal, and (2) you need to be able to squeeze every last Watt-hour out of your batteries (the EV equivalent of 'running on fumes').
Anytime you use a BMS you should be top balancing because that is what your BMS does.
The balancing process is basically taking slightly dissimilar capacity cells and 'balancing them' at either the top or the bottom so they either reach full or reach empty at the same time, but you cannot achieve both unless they were theoretically perfectly matched in which case only one would be necessary to achieve both. At least that is my understanding
Bottom balancing was supposedly popularized with EV's, but doesn't make much sense outside of that application and a select few others where (1) high C rates are normal, and (2) you need to be able to squeeze every last Watt-hour out of your batteries (the EV equivalent of 'running on fumes').
Anytime you use a BMS you should be top balancing because that is what your BMS does.
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D
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DZL, you answered soooooo many questions with that one visual.
I almost want to cry.
Thank you VERY much.
????
I almost want to cry.
Thank you VERY much.
????
DZL, you answered soooooo many questions with that one visual.
I almost want to cry.
Thank you VERY much.
I feel you! My own struggle to fully comprehend balancing for so long is what led me to make that visual aid, and helped it all 'click' in my mind. Glad its helpful for you and others too!
Also if its not clear from the image, the green line in the unbalanced pack is the reduced usable capacity due to imbalance. The red line is the usable capacity of a balanced pack.
terry82517
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Maybe a silly question but do you need a charger to balance cells? Or can it be done some way without? My battery will be charged via solar so don’t want to buy one Just to balance if I don’t have to.
You need a single cell charger or power supply. There are many good suggestions for parallel top balancing if you search the forums.
You need a charger to do it properly.
A less ideal alternative, but not dangerous as I understand it, is to skip the initial top balance step and try to let your BMS balance the cells over a long time period (weeks? months? forever? idk), so long as you understand that your pack will take a long time to balance and will have some amount of reduced capacity until it becomes balanced, and charge and discharge cutoffs (at the inverter and chargers) may need tweaking since pack and cell voltages may be out of step with eachother.
terry82517
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Thankyou for this. I saw that Will says you can let them self balance in his balance lifepo4 cells quickly video. My cells are new and it’s only a 12v 4s system so hopefully they wouldn’t take too long. Do you know what a acceptable difference between cells is? Sorry I’m very new and trying to read as much a possible on here but still have questions.
I think as important or more important than voltage and number of cells, is capacity of the cells and balance current of the BMS (as well as how well matched the cells are).
Sorry, I don't. And it would depend what you are referring to.
terry82517
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Personally it is what I would do, starts you off on the right foot. I feel your pain though (as do many others) its frustrating to have to spend money on balancing equipment that you may only use once or twice. Of course if you like to tinker, there are probably other uses you can find for a power supply.
An initial top balance is a good idea, particularly considering that you have large capacity 280ah cells. I've noticed a lot more people buying active balancers after buying 280 ah cells over the last few months, which gives me the impression people are finding their BMSes are not able to keep up with large capacity loosely matched cells. This is just my anecdotal impression based on what I'm seeing on the forum (and some comments Steve S has made about voltage differences upto 1-2 mV per Ah with the 280 EVE cells), I'm not sure if its accurate or not. But if it is, it makes an initial top balance even more important because your BMS will already be struggling to keep the cells balanced.
What BMS do you have or plan to buy? I can tell you the approximate balance current if its a common BMS.
terry82517
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One other use:
Some chargers try to be "smart" and won't output any current unless it sees a voltage on the line first. But if a BMS turns off the output FETs (say to protect the pack from low voltage), then the "smart" charger won't charge it -- and the BMS won't go back to normal operation until it gets charged.
The nice thing about power supplies is they're kinda dumb -- it just does what you tell it it. It's great! ;-)
That is a JBD Smart BMS, can't quite make out the serial number but looks like JDB-SP04S020 which is the 4S 120A version (like you said, one of the models Will recommends). I believe the balance current is in the ballpark of 50mA which is pretty low
terry82517
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