Charging Prius packs

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Tim writes asking how he could charge his 8S2P Prius battery pack used in an experimental EV. My Piaggio is 6S2P and I encountered numerous problems. Here was my response to him:

Using Prius packs are tricky and 8S2P is going to be even more difficult. Here are the problems:

  1. Charging voltage is very high

  2. Even "smart" chargers cannot detect Prius battery delta-V or delta-T to stop charging
  3. No parallel charging
  4. Packs expand on charging

* High voltages

Prius NiMH batteries are internally 6 cells, so a 6S pack is really a 36 cell pack, with nominal voltages of 43.2 volts (but usually hot off the charger at around 50 volts). An 8S pack would be 48 cells, so nominally 57.6 volts with a hot off the charge voltage above 60 volts.

Then the problem is that almost all commercial NiMH chargers go up to 50V (and even those are rare). I had trouble finding chargers for my 36-cell pack. The two I bought were: Astroflight 112D (NiMH version) and the "CH-UN4820 Multi-Current Smart Charger (2.0A) with 3 pins Plug for 36V or 48V NiMH / NiCd Pack."

You will most likely have to split your pack in half so you only charge 4S (24 cells) at a time.

* No parallel charging

First, you know you should never charge in parallel, so you need to add some connectors so you can charge as two 8S1P and 8S1P packs -- or four 4S1P packs. I recommend Anderson 75A power pole connectors.

* Smart chargers can't detect Prius delta-T/V

NiMH batteries are tricky to charge as there isn't a fixed final voltage as with LiIon/LiPo/Lead-acid. Instead, smart chargers usually look for a very small drop in voltage (delta-V) which signals the cell is full; or an increase in cell temperature (delta-T).

Unfortunately, Prius packs don't exhibit a measurable drop in voltage and don't increase in temperature when they are full. This is partly because of their pack design (prismatic rather than cylindrical). Instead, when a Prius pack is full, it bulges due to the increased internal pressure. Not good.

* Packs expand when charged

The prismatic (rectangular) Prius packs will swell and expand when they are overcharged. In order to prevent this, you have to design a strong battery holder that will keep good lateral pressure on them. My own holder (below) uses threaded rod and steel plates to hold the packs together. Even with 2mm steel, the packs have put enough pressure on the side plates to bend them and I'll have to make up new side plates sometime soon.

* How to charge

So how do you charge a Prius pack? I haven't found a smart charger smart enough to charge them using the standard delta-T and delta-V. So instead, I do what the battery management system in the Prius does -- I coulomb count. With my CycleAnalyst, I know how many watt-hours I've used in a ride. For example, for my commute I used 234.23 Wh which is around 5.90 Ah.

Knowing that my multi-current "smart" (stupid) charger has a nominal charge rate of 2.0 amps (and using a wattmeter), I set the charger using a timer to charge for 3 hours. This gives me 3 hours x 2 amps = 6 amp hours.

I have to repeat this twice, one for each side of my parallel chain.

I'm in the process of designing my own charge circuitry that will allow me to automate the coulomb counting. It will measure the output of the battery, then put the same back in (+10% for charging inefficiency). For a backup foolproof end-of-charge detection, I plan on using load cells to detect when the cells are expanding from internal pressure.

Of course, I'm busy with my day-job so this advanced Prius pack charger is still many many months (if not years) away given that a simple charger with a timer does much the same.

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Hey Karen,

Thanks for your super fast answer. Unfortunately it dissapointed me a bit (not your fault!) since i kinda realise now there is no 'fire and forget' solution for charging these prius puppies.

I think i will (for now) go with your solution of coulomb counting. Think i will use 4 laptop chargers (IBM 16V) for charging two batteries in series in combination with a watts meter and Cycle Analyst. Do you think its wise to build two 8module (42s) packs and parallel these or put 8 parallels in series? The reason i ask this is because two 8s in parallel requires only 1 blocking diode while 8 parallel packs make this more difficult. But on the other hand im brand new to this stuff so hopefully you can give me advice.

One more question -> whats the reaction of over-discharging a module? Will it simply shut down at a certain voltage putting out no more amps or will it decline to 0 volt and slowly die?
For some reason i cant log in with my google account to your blog. Facebook login also wont work. Suppose its caused by my explorer.

Thanks in advance for your reply!



First, I'm a bit hesitant to recommend your use of laptop power supplies. These are usually constant voltage, but not constant amperage. When you plug them into the packs, they may not react well. Either the laptop power supply will shut down or burn out, or it might try to pump either too little or too many amps. In any case, it'll be hard to monitor. I'd recommend a good constant-current (CC) power supply. You could hack up your own with some ICs, but I'm not sure if you make your own circuit boards.

Second, like most batteries, NiMH don't like to be run down. One of the cells might go into reverse voltage and refuse to be charged again. You should quit when the average voltage per cell is less than 1v. So for your 8S pack, that would be 48 volts.

As to whether 8S2P is better or 2 x 4S2P is better.... I'm not sure if it makes that much of a difference. Oh, one thing. You mention using a blocking diode. Note that if you are running around 100 amps like me, even with a 0.7 volt forward voltage drop, the blocking diode will dissipate 70 watts. This will produce a LOT of heat. Make sure to have a good heatsink or use a MOSFET as a "perfect diode" or figure out something else.


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This page contains a single entry by Karen Nakamura published on January 9, 2012 7:33 AM.

Programming an ATtiny w/ Arduino was the previous entry in this blog.

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