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Old 03-13-2015, 09:38 AM   #7 (permalink)
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Join Date: Feb 2010
Location: France
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Planetaire - '07 Toyota Prius 2 plug in
90 day: 195.47 mpg (US)
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Active bms. 13.2volts

There are two main bms familly:
-passive : bleeding
-active : capacitive, inductive, dc-dc transfer ...

I don't use bleeding because when cells goes up to 3.65V it is "too late".
Amperage during charging can really goes up to 100A and cannot be cut. With lifepo4 near 3.65v voltage goes up very quickly. May be a good diyer could fool the hsd and stop charging. On the 12v plus pole there is a sensing wire (a small one), just to read the battery voltage. May be if a device increase this voltage the hsd will stop charging the 12v battery ?

So I use an active bms, a capacitive one, that transfert energy from the higher cell to the lower. It does this permanently, 24h/24h 365 days/year.
So when I drive my prius, at the begining of each trip the 4 cells are always balanced, often within only 0.01 volts difference between the 4 cells.
There is a small voltage difference at the begining, just during the initial charging and after, during the trip, the bms rebalance them.

In winter you have to be careful. Some lifepo4 may be charged only over 0C, others under.
And charging amperage can goes up to 100A (I have seen such an amperage with a lead acid battery in a prius), so in winter avoid discharging your cells during power_off or accessories positions.
Also check that you cells accept such an amperage. With 20Ah cells that is 5C. Usually it is 1C and more during few seconds, but you have to buy cells with 5C charging, it could happen.

13.2 volt is the rest voltage, in power_off mode. When well charged (and they are, 3.52v during power_on is near 100% charge), after waiting several minutes, the A123 have 3.3v. Others lifepo4 have 3.2v.

Low voltage cutoff may be worse then not having one. It must be 100% safe and as you say it can use some energy and drain the cells.

I was only talking about the gen 2 Prius. The gen 3 can have a highter voltage during charging (something like 14.7 volts ?)

May be, if one of your cells don't have the same capacity or was not charged as the 3 others, may be its voltage goes to 0 volts and not the 3 others cells.

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