# Lead Acid BMS



## Tesseract (Sep 27, 2008)

Hey drewcg and welcome to this _mostly_ well-behaved forum.

Leaving aside any comments/opinions as to the market-worthiness of this product, there are a few practical considerations:

1. Lead-acid (PbA) will self-balance ("equalize") to some extent. Now, if one battery is sitting at 10.5V (0% State of Charge, or "SoC") and another is at 12.9V (100%) then you won't be able to self-balance them, but the AGM pack on my dyno setup seems to self-balance with up to 1V of difference between batteries.

2. PbA batteries come in 3 different common voltages (6, 8, 12) while LFP cells only come in one voltage (well, there is some difference between Sky Energy and ThunderSky cells, but we're talking half a volt, not several volts).

3. The shunt (balancing) resistor has to withstand the total voltage across the cell/battery times the shunting current. For an LFP cell the shunting voltage will typically be 3.65V to 3.85V; for a PbA battery it will be 7.2V (6V), 9.6V (8V) or 14.4V (12V). If the shunting current is about the same for each, then you can see that the resistor for the PbA balancer has to be much larger than the one for LFP for the same amount of shunting current.

4. Rarely do you have access to the individual cells in a PbA battery, but it's an individual cell going bad first that typically dooms the entire battery.

5. The relative cost of the BMS vs. the battery/cell needs to be considered. There is no obvious winner here, but I point this out anyway because most people won't spend any more than $20 on a BMS board for a $100 cell/battery.

6. Finally, there is considerable debate whether balancing during charging is even desirable for LFP, but the consensus is that there is merit in doing this for PbA (at least to prevent any battery in a string from exceeding the normal "equalizing" voltage, which can be as high as 15.5V for a 12V battery). So that's a plus, anyway.

Anyway, you might want to check out the MiniBMS project by fellow member dimitri to see a very economical implementation of a cell-level BMS with pretty much all the features necessary for most folks (it even applies some temperature compensation to LVC).


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## drewcg (Oct 6, 2009)

Hi Tesseract,

Thanks for the input. The idea for the post was to get an idea of what people would be looking for in a BMS dedicated to lead acids. There are tons of DIY PbA conversions out there and I have met a few people who want more information on their pack than just open circuit system voltage. I have a 96v PbA car with a dumb charger and over they years have wanted a way to collect metrics on the pack, mostly for personal interest.

Since PbAs can be equalized with a fixed overcharge, load resistors used for passive balancing on lithium chemistries don't seem like much a desired or useful feature. Now, since SOC on PbAs can be determined by open circuit voltage a smart BMS with digital communication could be useful for getting better SOC than with just a meter. Further, if a BMS had a processor then all sorts of information could be collected such as internal resistance, lifetime mins/max, battery health based on load/unload rise/fall times, etc. 

Drew


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