Battery Pack Voltage Question from rookie

[RobW]

I need some advice from some of you experts out there, because there is no-one around my area that I know of that has any dealings with this sort of thing. So I have to rely on you guys for some advice/help.

I am a rookie EV conversionist, meaning that I know just enough to be dangerous, and even less about electric vehicles.

I have a question about battery voltage. I have converted a 1983 Volvo 760 to electric. The other day when I test drove the car it seemed to be somewhat peppie when I would accelerate. After a few test drives back and forth through the yard, I knew my voltage had dropped a bit.

The car seem to be losing power and not as peppie as I thought it should for no further than it had been driven. So I decided to check each battery to see what the voltage was

I am using “6” - 12v truck batteries,. Three of my batteries are somewhat week, so I knew they would be a bit low. Battery one (negative post connected to B-) was 11.2v. Battery two was 2.8v. Batt three was 11.4v, and then the last three were above 12v on each one. That meant the total voltage was approx 60volts.

I guess my question is: how much of a negative effect would that one battery at 2v have on the total system? If I can’t even get a mile out of them, then I sure can’t risk getting out in traffic.

I am using the 144v open revolt controller
Thanks for your advice. (Unless you tell me to take it to the junk yard) 

[mora]

I'm no expert, but your battery #2 is probably toast. You've drained it too empty or it could have been bad already when you got it. Any of your batteries shouldn't be anywhere below 10V. Remove bad battery from series string or replace with better one. Your performance is directly related to your weakest battery.

How do you charge your batteries? Do you have one 72V charger or 12V charger?

[RobW]

Right now I am charging them one at a time. I haven't bought a charger/chargers yet. I was thinking about using a charger for each battery. That battery that checked at 2.8v I took out and gave the car a try at 60v, but no real difference in performance. It seems the first test drive around the yard it had good getup and go, but now kinda sluggish. I was trying to get the precharge on the controller working before a second test drive around the yard. I am having issues with something in the controller and the precharge. I would assume that doesn't have anything to do with the sluggishness. Right?? BTW, I am communicating with Paul Holmes about the p-channel mosfet etc.

[mechman600]

Definitely charge each battery individually so you know 100% that they are all fully charged. Then let them sit for a day to check resting voltages. If you have any that are connected in parallel (which it doesn't sound like you do), remember to separate them.
They should all be within 0.05V of each other at the end of the day. I would hazard a guess that battery number two will not be the same as the others, but you never know.

Even if they are all the same voltage, I have had it in the past where all batteries rest at the same voltage but one or two simply have next to no capacity once you start working them. I learned the hard way from using used batteries!

If battery two continues to crap out quickly after a full charge once you start working it, it's toast.

Good luck!

[puddleglum]

Do you have a way to load test your batteries individually and/or test the voltage of each battery while it is under load in the car? If you are using used batteries, even if the batteries all show fully charged, they may be way different under load, as mechman said. You should make sure your batteries are good before looking further IMO. If there is a current cutback setting in the controller, it may be shutting you down if the battery voltage is falling to low.

[mechman600]

Yes, monitoring each battery voltage under load while you drive very quickly sorts out which batteries are toast. I think I bought six volt meters on eBay for $24. You can wire them to your pack with only 7 wires (instead of 12 like you would assume).

This is what I use:

Plugged into the car with this connector:
[IMG]http://i1119.photobucket.com/albums/k634/mechman600/54e56760.jpg[/IMG}

[RobW]

Thanks everyone! I found that two batteries were discharging very very fast, so they have been replaced!

[MPaulHolmes]

Originally Posted by mechman600

Plugged into the car with this connector:

.

[Ryland]

Your pack is only as good as the weakest battery, that is why people try to get a matched set, even if they are used.
If you had kept that wore out battery in the string it would have been worse then removing it.

Are they flooded or sealed batteries??

[mechman600]

Thanks, MPaulHolmes. Forgot to mention that connector, which is readily available if you frequently repair Caterpillar diesels.