PHEV Battery Failures…

I had four of the PHEV 18Ah batteries fail last week.  The first swelled up around one cell so I knew it was bad.  Strange thing is it happened during discharge and not during charging.  I got it replaced under warranty with a new one. I was doing my Saturday individual battery voltage check the following Saturday and I found one battery only charged up to 11 volts so I took it back and got it replaced. Two days later I found two more only at 11 volts.  Seems I was suffering from the classic 20 battery in series charging problem, some batteries were not fully recharging overnight.  I put one on a nice slow regulated charger and it did recover to a full 13.8 volts in two days.  The other did not so I did the only thing I could think of, slapped a 60 watt 12 volt bulb on it and left it to die overnight.  The next morning it was stone dead, I put it on a nice slow 1.5 amp charge and left it on a timer for 6 hours.  It is now back to 13.8 volts as well.  Seems it takes a lot of current to get the reversed cell to come back and the only way to do it is to discharge the other 5 cells to zero volts.  I know this is not recommended but faced with a 11 volt battery I really had nothing to loose.

Now to fix the problem.  I came up with a setup that I am glad to report is working very well.  With the suggestion coming from the eaa-phev list I rounded up some zener diodes.  I used a small 3A blade fuse holder, a 6.2V 5watt zener, a 8.2V 5watt in series across each of the 20 batteries.  I mounted the fuse holder with hot-melt glue between the batteries.  The fuse can be easily accessed from the top where you can actually put an amp meter across the fuse socket and measure the bypass current across the battery, way cool. The 3 amp fuse acts like a .3 ohm resistor.

I have put together a kit of all parts and offer it for sale for just $2 per kit is anyone is interested. (Well below the retail cost of these parts) If you want pre-cut orange and brown #16 TFFN wire the kit is just $2.25 for the wire included.  The zeners fit nicely inside the crimp sleeves, no soldering required. One kit required for each battery.

The kit contains:  one 6.2volt 5 watt Zener Diode, one 8.2volt 5 watt Zener Diode, one 3amp blade fuse, one enclosed fuse holder, two female crimp connectors, two crimp ring terminals, two crimp butt splices. (two pre-cut wires).

I will be adding assembly instructions here soon.  I will assemble if you don’t want the hassle for an additional $1 per kit.

10JUN2008 report.  All 20 batteries fully recharged overnight, the Zener Diodes equalized the pack nicely.  I know a BMS system is the ultimate answer but until I get my design finished I highly suggest the interim Zener Diode Shunt system for anyone using Lead-Acid Packs.

 

 (to be continued…)

 

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