Balancing the Battery Pack

As you may recall, what originally tripped the Prius error was what basically to a voltage difference between the modules. The sensors in the battery pack report the module voltage (I think in groups of 2? You can see the wires leading to a contact in each section of the bus bar), and the ECU will report an error beyond a certain threshold. I believe it’s a difference in anything greater than 0.3V between any two module groups that will trigger an error, but I’m not positive that’s the threshold.

The significance of this is that cycling the batteries isn’t enough to “reset” the pack. The voltage also needs to be balanced across all the modules. The last charge of the modules left it at a relatively arbitrary voltage, as the charge terminated after reaching the 7250 mAh mark. Here’s what I ended up with:

Module Voltage
1 7.96
2 8.01
3 7.97
4 7.96
5 7.95
6 8.04
7 8.02
8 8.03
9 7.95
10 7.93
11 7.94
12 8.00
13 7.97
14 7.97
15 8.03
16 8.03
17 7.92
18 8.00
19 8.03
20 7.95
21 8.09
22 7.99
23 7.98
24 8.04
25 7.98
26 8.05
27 7.93
28 8.03

So the lowest voltage is 7.92V and the highest is 8.09V. That yields a difference of 0.17V – far beyond the tolerance. The modules as designed to drive the Prius are hooked up in series – 28 * 7.2 nominal volts per module = 201.6V. That’s the dangerous part. Welding level voltages. To balance all the modules we hook them up in parallel. One ridiculously giant 7.2V battery, probably run your cell phone for a couple years. This part is not that dangerous. You could have quite a bit of current, but the voltage is low enough that you’d have to trip and accidentally impale yourself on a couple of the contacts for it to really matter.

I read about a couple strategies for doing this, but the wiring harnesses didn’t look very robust, easy, or cheap. I really didn’t want to worry about all those wires. It would be extremely easy to accidentally hook up a module backwards and ruin it. Given that removing the bad module had been relatively easy, I opted for a different strategy.

Flip every second pack, so the terminals are all aligned:


Then run a wire across:



This worked wonderfully! And it was super easy to avoid screwing up, which is a big perk in my book. I left it like this for a little over a day, and it all seemed to work out. The pack equalized at (I didn’t actually write this down, so just going off memory) 7.96V and all good to go!


16 thoughts on “Balancing the Battery Pack

    1. tobymurray Post author

      It’s still going strong. It’s been going for a solid 60 000km, I’m not exactly sure because I don’t recall what the odometer read when I replaced it. So far I haven’t noticed any issues at all, and the car is getting old enough that I’m hoping the battery will last right until the end…

  1. Joshua Wallace

    first of all fantastic site very informative and thank you! so you just run a wire across all points, is that negative side first and then positive side? just double checking on how not to get zapped 🙂

    1. tobymurray Post author

      Just to be super clear the big safety concern here is that you connect them all in parallel – i.e. all the negative ends are connected to one another and separately all the positive ends are connected to one another. If you connect them in series, like how the mount in the car, you can seriously hurt yourself by accidentally shorting the circuit.

      That said, you have it right – after flipping every second pack so they’re all the same direction I just ran a bare copper wire the whole length and tightened down each of the nuts individually. After tightening them down, I did a second pass to be double sure they were all touching securely. It shouldn’t make any difference whether you connect all the positive ends first or the negative ends first, from either an effectiveness perspective or a safety perspective.

      If you’re balancing a hybrid battery pack, I’d love to hear how it goes for you – please update or leave a link if you’re writing about it!

  2. nick

    hey i was curious if you could drop me a line. I am trying to figure out the best way to rebalance my battery pack what type of charger did you use? and any pointers. etc.

  3. Mark

    I have a second generation Prius and have been doing some research in regards to replacement/reconditioning/balancing options. Once I have the opportunity to determine whether I will be replacing were attempting to recondition. I would like to use this method to complete the balancing process. The instructions seem to be thorough and pretty straightforward but could you please add what type of equipment you used to complete the process. Thanks

    1. tobymurray Post author

      Are you referring to specifically the equipment used for this single step? I added links in other steps (see the first few for buying decisions) for anything I bought. I didn’t link things like a multi-meter (the one I have is whatever the cheapest thing on Amazon was at the time). There was no equipment of interest used in this balancing step. The mentioned multi-meter, a single socket for undoing and redoing the nut on each terminal, and a length of uninsulated copper wire. I believe this particular wire was left over from a length of grounded 2-conductor cord I’d cannibalized for a different project, but you could use anything.

  4. Dionisie

    Hi, very interesting.. I have so many packs and I’ve been trying to find the best way to balance them, someone told me about that method, linking all modules in parallel leaving the pack for 6 hours and the result was brilliant.

  5. Henry R. Quiroz

    Hi so my question is does it balance out itself when you connect them together parallel or is there a charger that you charge them with is there any kind of charger connected to the battery cells at all?

    1. tobymurray Post author

      The short answer is that the pack balances itself out when left with everything connected in parallel. They should be pretty close to each other in terms of voltage before you do so, this is just for aligning in the final few tenths or hundredths of a volt.

      Charging a battery generally takes place by attaching a higher voltage power source to the lower voltage battery. The difference in electrical potential will cause the power source to move energy into the battery naturally. As long as there is current available and a voltage difference, electricity will flow. When you’re balancing the battery pack, the power source is just the other batteries. If one is at 8.00 volts and one is at 7.95 volts, left long enough the higher voltage one will drop and the lower voltage one will rise. They won’t necessarily meet in the middle, but left long enough they could all end up at e.g. 7.97 volts or something in that example.

  6. Cliff

    Folks, when rebalancing the battery pack, you don’t need a charger. In fact, I suggest absolutely NOT connecting a charger unless it’s designed for 8v.

    Here’s why connecting all the positive terminals together and all the negative terminals together will rebalance the battery. When all the cells are connected together in parallel like this, the modules with the higher voltage will act like a battery charger and the modules with the lower voltages will accept the charge. After a certain amount of time, all of the battery modules will approach the same voltage (or very,very close to it.) because the modules with the higher charge will discharge and the low voltage cells will charge to an equilibrium voltage.

    Once the pack is rebalanced, reassemble the modules properly and put it back in the car. Use the car to recharge the entire battery pack to operational levels. It’s the safest way to do it.

    1. tobymurray Post author

      Agreed, charging/discharging the battery is effective for finding under-preforming cells, breaking any “memory” that may have developed and validating the modules perform as expected across their expected capacity. Its only use when it comes to balancing might be to bring the cells closer together so the balancing doesn’t take as long.

      Definitely one should not try and fix anything via charging if they don’t understand what they’re doing. Balancing alone will not address the memory effect that NiMH batteries can have. While not as prominent as some other chemistries, the effect is still a consideration (particularly for old, heavily used batteries like those in a hybrid!)

    2. tobymurray Post author

      Also worth noting – a big voltage difference between a module and the rest of the battery can wreck the module. While balancing the battery that doesn’t necessarily mean charging the module, it may be more convenient than safely discharging the rest of the battery. The modules should be in the neighborhood of being balanced before finally being connected together.


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