Wednesday, February 4, 2009

Battery Imbalances

I've been somewhat concerned with how imbalanced my pack has been lately. Given that the charging system has been blown for a few days, I'm not too surprised.

Here are some observations. The front two batteries behind the radiator grille are the lowest voltage in the pack. They tend to be the coldest because they get lots of air while I'm driving. They also get more drain then other batteries because they supply 24V to the controller and to the contactor. Furthermore, one of the front batteries is tied to the IsoBatMon unit. The IsoBatMon pulls no power when there is no LVP/OVP signal activity; however, when charging, it pulls 10mA when the OVP signal is on. I think the voltage drop across the zener diodes on that BatMon board is just a few tenths of a volt higher than the other BatMon boards (home fix job).

Thus, we have a perfect storm happening on the front-center battery that gets the highest load, and equalizes last due to a slightly different zener drop on the BatMon board. I think I'll swap boards with another battery to see if that lets the green light come on earlier.

In the end, the pack imbalance isn't horrible. If I take no-load voltage measurements on the batteries after they sit for an hour, the voltage difference is less than 150mV, so maybe my expectations are too high for what kind of balancing to expect.

Anyway, I'm just sharing these thoughts so they don't get lost in time. Cheers.


george said...

I've been lurking your blog for a week or so now and finally found a chance to comment.

How viable would it be to install a wind guard of some sort in front of those low voltage batteries to protect them from direct wind chill. And I'd also think installing a controllable vent in the grille would make sense in your climate. Something like you see in air registers that's mechanically simple and remotely activated using a mechanical cable pull switch.

Maybe sealing the front compartment as much as possible. Then controlling airflow depending on the ambient temperature and the compartment temperature.

TimK said...

Hi George,

Yes, that's a great idea. Bryan at Belktronix suggested the same, including adding styrofoam or some other insulator to protect the front batteries. I'm a bit tired from the hard push over the past six months to get this done, so I'll definitely consider that soon after a short break. Tim