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Shunt Equalizer Test.

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waitatian:
Shunt Equalizer Test.
There is a problem with charging series connected batteries. They may start out the same, but they can age differently,and a new replacement battery added to a string is not the same as the rest.
When series connected batteries are charged, the regulator will eventually fix the system voltage, and each battery should have an equal share. If the voltage on some of the batteries goes above their share, the voltage on the rest of them must go down because the total voltage is fixed. Equalizing charges are meant to keep them the same but if the batteries are not equal, no amount of overcharging will make them so.
My house supply is a 48v string of mixed age 6V US2205. When there was plenty of charging power, two of the batteries can be getting overcharged at 7.8v at the same time as two others are being undercharged at 6.8v. To fix this it was either a new set of batteries or some form of active equalizing. I tried a shunt equalzser.
It works by comparing the voltage on two adjacent batteries. A resistor bleeds current off the higher voltage one.
I initially tested it on my 24v shed supply. For the test, I set it up with two US2205 about 1 year old and two about 5 years old. Charging was by four 12V, 40W panels in series through a cheap Chinese MPPT regulator.
  After a month of settling in time, during charge the voltage on one battery was high, correct on another and low on two. So one was overcharged while two were undercharged. The equalizer was connected on a sunny day. One battery was immediately bypassed by about 3A - the maximum the equalizer will do. Over about an hour, as the three remaining batteries got more charge, a second battery started to be bypassed. It took about a week for three batteries to be bypassed, meaning most of the charging current was going into one battery. Over about a month, the bypass current went down as they were more equally charged. Now on float, only about 100mA is being bypassed across 2 batteries, and all the batteries are within 10mV of each other during charge/float.
 The house supply was then set up and a similar equalizing pattern happened, except at the end 6 batteries were being fully bypassed. This means the two un-bypassed batteries were needing a few amps of extra charging current at all times to keep their voltage the same as the rest.
 Load testing the two batteries showed reduced capacity, but they could still provide power. The equalizer allowed them to get charged without ruining the rest of the batteries, and they could stay in service until I could afford to replace them.
The two low batteries remained in service for the summer with no problems, and were replaced with new ones. Due to poor weather, it took about 3 weeks for the new batteries to settle in. Now, only the batteries on each end of the supply have any serious bypassing and a few others are being shunted by a few hundred ma.
 There were no problems with the Chinese MPPT controller or the Tristar PWM regulating the house supply.
Shunt regulators waste power when bypassing so in poor charging weather some of the charging power will be wasted.
This may stop the batteries being fully charged, but they will be equally not fully charged.

waitatian:
Some pictures.

SparWeb:
Thank you waitatian for sharing this!
I haven's seen a shunt equalizer (or anything like a BMS) on Fieldlines for a long time.
I think it was another kiwi who posted it, then, too.

I have a similar problem with some of the batteries in my series string - they are always a few % weaker than the rest.  It's not enough to kill the stack, but every year the split between the best and the worst gets wider.

My wind diversion load is going off frequently, so wasting some energy to this form of balancing would not be a concern to me.

Is that a PIC on each board controlling the equalizing current?

waitatian:
Hi,
It is an op-amp.Doing it with microcontrollers would make it more efficient and probably cheaper, but I needed a quick solution. I could dig up the circuit and post it. It's an op-amp in differential mode driving a darlington transistor/resistor on each battery. The equalizers have been going over a year with no problems.

SparWeb:
I'd love to see the schematic!

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