Monday, November 17, 2014

Rewiring the Battery Box

As mentioned before, the lead acid batteries that power the house has seen better days.  The batteries being used are 24 6V batteries, wired in 3 parallel strings of 8 batteries with system voltage of 48V.  I was not able to read any direct model numbers off the batteries, but they probably have rated capacity of 300~350Ahr.  The actual capacity at time point is probably significantly lower, though.

Lead Acid Batteries

Below is the actual wiring diagram of how the system was originally wired up.  The three parallel strings are made up of batteries 5~12, 17~24, and 1~4,13~16.
Battery Pack Wiring
Due to the physical sizing of the battery box and the fact that these are 6V batteries, something had to give, and in this case, it was how the three parallel strings are wired up.  Best practices for battery sizing seems to guide people to minimize parallel strings to reduce possibility of imbalance.  In fact, the ideal battery configuration seems to be 2V x 24 (or however you may need for target system voltage) or just one series string of batteries.  All I've read seems to heavily discourage 2+ parallel strings, and if you end up with a 3+ parallel batteries, not to tie them to each other, but to run similar length wires individually to each parallel string and tie them together at a common point.

As you can see above in the wiring diagram, this was not how the system was wired.  The 1~4,13~16 battery string is wired using a wire that's more than 6 feet long meaning it sees significantly higher resistance than the other two strings.  I tried to top up the batteries using a generator and made voltage measurements at rest and under load, which came up as follows:

At Rest:
6.18V 6.18V 6.07V 6.16V 6.37V 6.36V 6.40V 6.33V 6.38V 6.33V 6.35V 4.61V
6.20V 6.16V 6.16V 5.97V 5.83V 6.18V 6.25V 6.19V 6.21V 6.20V 6.07V 6.11V

Under Load (~C/20)
6.04V 6.13V 5.88V 6.01V 6.25V 6.25V 6.27V 6.20V 6.23V 6.24V 6.25V 2.40V
6.14V 6.13V 6.08V 3.50V 3.80V 6.07V 6.11V 6.06V 6.06V 6.09V 5.93V 5.99V

Red colored readings mean that the battery's basically dead, and the orange means it's compromised.  If I overlay the battery information on the wiring chart, I get the following:

Compromised Batteries

You can see that the dead or compromised batteries are the ones closest to parallel wiring, where it was likely seeing significant amounts of overcharging and overdrawing during the charge / discharge cycles.  The long term solution would be to replace the battery pack with properly designed one, but that has to happen after we had more solar generation capacity - currently, the solar panels are not quite large enough to fully charge up the battery pack even on a sunny summer day.

For the time being, I simply rewired battery pack to get us through the winter - I pushed out the batteries 3, 12, 16, 17, 23, 24 to the end and removed the 1~4,13~16 string.  The battery pack now has two parallel strings, with the parallel wires being doubled up for extra current carrying capacity.  The two parallel strings now go positions 3~10 and 15~22.  At +100lb, this was surprisingly good workout. :-)

The system voltage is now in the correct range instead of swinging between 45V~47V.  Unfortunately I forgot to bring the Bluetooth serial module that I was going to use to read out and store the battery monitor information.  Hopefully I'll remember next time I head down!

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