Convert Dakota 36V to 12V

[Jasonoski]

I was given a 36V Dakota 60ah LiFePo4. Was told the owner bought it new, used it once and then put a lead acid charger on it. The over voltage burned out the BMS and it registers no voltage at the terminals. Dakota won’t warranty it so he gave it to me.

I would like to use the cells to build a 12V battery as all my current needs are 12V.


First question:
Should I just replace the BMS and use a 36V to 12V converter or reconfigure the cells to 12V and use a 12V BMS?

I’ll need BMS recommendations too. Must have Bluetooth, temp cutoffs and heater control since I use my batteries in Wisconsin winters.

Here are pics of how far I’ve gone into it. Advice on tear down is also appreciated since this is my first time opening a LiFePo4.

Thanks,
Jasonoski

 

[sunshine_eggo]

What voltage do you measure from the terminal indicated in red and the (+) of the battery?

 

[Jasonoski]

I measured 40.3 volts. Sunshine_eggo, thank you for the clear indication of where to measure.

 

[sunshine_eggo]

I measured 40.3 volts. Sunshine_eggo, thank you for the clear indication of where to measure.

Assuming the cells are consistent, that would put all 12 cells at an average of 3.36V - very charged.




This glob is likely the BMS sensing wires connector. In some cases, unplugging this connector for 30 seconds and reconnecting may reset the BMS. You would need to carefully remove the snot without damaging the connector, wire or BMS.

This connector can also provide a means of measuring individual cell voltages to confirm they are in a healthy voltage range. Once you've established that, then you can make a sensible decision on reconfiguring to a new voltage or simply replacing the BMS.

 

[Jasonoski]

I carefully removed the glob and disconnected the connector. Waited 20 seconds and reconnected. The battery now reads 40.2 volts at the external + and - battery terminals. It used to read .05 volts. Thank you sunshine_eggo for the guidance.

Is it reasonable to assume the BMS reset successfully? Are there additional checks I can make to verify the BMS doesn’t have any residual problems from the overcharge?

I don’t currently have a 36V charger. All my camper and stationary battery systems are 12V.

 

[sunshine_eggo]

"over-charge" is essentially impossible with a functional BMS. The BMS does its job, disconnects the battery charging circuit and prevents additional charge.

It is concerning that the BMS was still in some sort of protection mode that also prevented discharging as that would be the normal way to resolve it, i.e., charge protection is enabled, but the battery still reports voltage and allows discharge to bring the voltage down to allow charging again.

Putting it on a 48V charger could conceivably over-volt the BMS and damage it, but in that case, I would not expect the BMS to ever work again.

FWIW, back to your original post, 36V LFP is nearly identical to lead acid voltage-wise in terms of the total voltage range, so using lead acid chargers on LFP is not necessarily a bad thing.

Testing would be prudent. Repeat charging to 43.8V (might need to go to 45V) or higher should trigger BMS protection again and cut off charging. I would confirm that happens.

The BMS should also stop discharge somewhere around 30V, so it would be a good idea to confirm that works as well.

Once you've confirmed the BMS functions at both limits, I would say it's okay to use it normally.

 

[Jasonoski]

Thanks for guiding me through getting the BMS functional again. I don’t have a 36V charger so I probably won’t do further testing of the original BMS.

My onboard Victron DC to DC charger is 12V and both my portable chargers are 12V (40 amp and 110 amp) so I’d like to reconfigure the cells to build a 12V battery. I have multiple uses for 12V units to replace current lead acid batteries on a stationary camper and a remote water pump. I just charge them with a generator when solar isn’t sufficient.

I’ve read about and watched videos on other LFP tear downs. I’m assuming I should start by testing the cells individually in place and then remove the whole battery assembly from the housing.

How should I go about initial testing of the cells while still in place?

Thanks for all the guidance so far,
Jasonoski

 

[sunshine_eggo]

Thanks for guiding me through getting the BMS functional again. I don’t have a 36V charger so I probably won’t do further testing of the original BMS.

My onboard Victron DC to DC charger is 12V and both my portable chargers are 12V (40 amp and 110 amp) so I’d like to reconfigure the cells to build a 12V battery. I have multiple uses for 12V units to replace current lead acid batteries on a stationary camper and a remote water pump. I just charge them with a generator when solar isn’t sufficient.

I’ve read about and watched videos on other LFP tear downs. I’m assuming I should start by testing the cells individually in place and then remove the whole battery assembly from the housing.

How should I go about initial testing of the cells while still in place?

Thanks for all the guidance so far,
Jasonoski

Not much you can do besides check voltages.

Using a very fine wire like a twisty-tie into the pin holes, you could measure the voltages between the pins on the BMS connector. The 13 holes are for the 12 cells.

Easiest is to start at pin 1 and pin 2, measure voltage. That should be the voltage of the first cell.

Then move pin 2 to pin 3 and measure pin 1 to pin 3. That's cell 1 + cell 2, and you're only futzing about with moving one wire rather than both.

Wire 1 to 2: cell 1
Wire 1 to 3: cell 1 + cell 2
Wire 1 to 4: cell 1 + cell 2 + cell 3, etc.

You can then subtract out the relevant cells to get the cell of each.

Or you can simply move the wires to the next two adjacent pins. Whichever is easier for you.

 

[Jasonoski]

I started with pin 1 and 2 and they measured at 3.34V

Did the next two pins, etc. each set of pins was 3.34V

So that should confirm all the cells are good or at least reading good voltage.

I think my next step is selecting a 12V BMS. Here are my criteria:

-High and low voltage cut off
-High and low temp charge/load cut off
-Bluetooth
-Charge amps 40 minimum 150+ preferred
-Load amps 40 minimum
-Low temp charge heater
-Multiple temp sensors (if possible)
-Reliable

Guidance and recommendations would be greatly appreciated.

Thanks!
Jasonoski