# how do i check for a dud/weak battery.



## subevo (Aug 23, 2008)

i have a 72volt lead acid pack of 6 batteries at 12volts each.i suspect one is playing up.what can i do to find the culprit?


----------



## Jimdear2 (Oct 12, 2008)

subevo said:


> i have a 72volt lead acid pack of 6 batteries at 12volts each.i suspect one is playing up.what can i do to find the culprit?


Flooded?
AGM?
Gels?


----------



## Zemmo (Sep 13, 2007)

A load tester and a volt meter will tell you a lot of info about that one battery.


----------



## rmay635703 (Oct 23, 2008)

subevo said:


> i have a 72volt lead acid pack of 6 batteries at 12volts each.i suspect one is playing up.what can i do to find the culprit?


A load tester won't always do it, best to drive the car a SAFE distance then break out your trusty $1.99 harbor freight digital multimeter and check each battery, if one is significantly lower than the others its probably a dud.

Cheers
Ryan


----------



## CPLTECH (Sep 14, 2007)

rmay635703 said:


> A load tester won't always do it, best to drive the car a SAFE distance then break out your trusty $1.99 harbor freight digital multimeter and check each battery, if one is significantly lower than the others it's probably a dud.
> 
> Cheers
> Ryan


I like the advice Ryan just gave but would go one step further.

After that short trip, turn on the 12V accessories, such as headlights and fans (assuming it uses a DC-DC converter). That way the whole battery string will see at least a “slight” load so you can find the culprit(s).


----------



## Zemmo (Sep 13, 2007)

A load tester would be the same thing as driving the EV but you can do it without moving. A load tester puts a 100 amp load on the battery and you measure the voltage drop. I have a battery right now in my EV that measures weak open circuit but under the load, it flat lines to 0. I put a DVM on that battery drove around the block and under driving conditions in the battery string that battery went from weak battery to -7 volts. So it was even worse yet because of reversing. So a load tester does tell you a lot of good info about your battery.


----------



## subevo (Aug 23, 2008)

thanks i forgot to say its flooded batteries ive got.was going to buy a hydrometer but will try the above method.


----------



## rmay635703 (Oct 23, 2008)

subevo said:


> thanks i forgot to say its flooded batteries ive got.was going to buy a hydrometer but will try the above method.


Actually a hydrometer is optimal, you will find a few cells that are plain DEAD in a bad battery.

Good way to find shorts.


----------



## Jimdear2 (Oct 12, 2008)

Thats why I've been waiting to find out what kind of batteries before I reccommend how to test. 

For flodded I would say a hydrometer is the best way. 

For sealed lead acid you can do voltage or current testeing depending on available equippment.

Since your batterie are flooded types, charge them up, bleed off the surface charge or let them sit for at least a few hours then test the specific gravity (s.g). for each cell. You will soon find out total pack condition. Any cells that vary more then a few points from others in the same battery are suspect

Remember that flooded batteries can benifit from balancing too. If you find one batttery with low but even s.g. try slow charging that one battery. If the s.g. comes up you just might have a lazy battery that will need balancing every so often. Or maybe that one kick in the pants up to full charge will be all it needs.

Hope this information is useful


----------



## subevo (Aug 23, 2008)

with my paktrakr i have now discovered that one of my batteries is dropping to 3.5 volts after 6 miles of driving.strangely on the way home after a charge its ok for the full 9 miles home showing 10.7volts under heavy load.anyone else experienced this.why does the battery drop so low.ive done a hydrometer check and all read ok except one cell which is only a tiny bit lower but still in the green zone.ive swapped the sensor wires and confirmed it definetely is that battery.when fully charged battery reads 12.8volts with multimeter.i find this bizarre.


----------



## Jimdear2 (Oct 12, 2008)

subevo said:


> with my paktrakr i have now discovered that one of my batteries is dropping to 3.5 volts after 6 miles of driving.strangely on the way home after a charge its ok for the full 9 miles home showing 10.7volts under heavy load.anyone else experienced this.why does the battery drop so low.ive done a hydrometer check and all read ok except one cell which is only a tiny bit lower but still in the green zone.ive swapped the sensor wires and confirmed it definetely is that battery.when fully charged battery reads 12.8volts with multimeter.i find this bizarre.


I would suspect that you have some temperature related internal connection/resistance problem. 

I did have a battery that gave similar symptoms. When charged or discharged at under 45 amps it was fine. When I applied a load of over 50 amps the battery would fail, the voltage would drop. After it cooled for a while the battery would be fine again (up to about 45 amps).

The way yours is reading , I suspect that:
1.) you are going uphill in the morning and down hill in the evening
OR
2.) It's cold in the morning and warm in the afternoon.

Just my best guesses.


----------

