my golfcart uses 12V36Amp battery......if i use 2 new 12V18Amp batteries hooked in parallel (12 v36 amp) I get less useage than a single 12V36Amp battery...Why?

 

They hunt each other.

They discharge through each other so that they are only as good as the weaker of the two and the tend to get weaker faster.

You normally want to isolate the batteries from each other.

 

i have isolated the batteries and i get 9 holes out of each one.....so the batteries are equal.....yet out of single 12v36 amp i can get 27+ holes why?

 

When you say 36A, I'm assuming you mean 36 amp-hours (Ah). Are you using the same brand/type of battery just a different Ah rating? Are you sure both batteries are fully charged? If one of the batteries isn't fully charged and the other is, the lesser-charged battery is going to drag down the fully charged one, thereby decreasin runtime.

 

The evidence you've given seems to say the batteries may be different. The best test would be to charge the batteries to the normal levels, then separately discharge them through a known load (e.g. a DC load) to determine their true capabilities. After that is done, then you can address issues like the batteries not being isolated, etc.

Otherwise, you don't know if the difference is due to the batteries, charging method, or connection method.

Lacking the tools to do that testing, you could take the batteries to a business that has one of the microprocessor-controlled battering testing instruments and they can do this work for you. But of course you'll have to pay them some money to do this.

The nice thing about something like a DC load is that you can e.g. set it up to draw a constant power or current (and, if you wish, do it intermittently to simulate use). Then you can run the test until the battery's voltage drops below a certain level -- or a specific integrated current has been reached. Unfortunately, most folks don't have a suitable DC load. You'd probably want one rated to 0.5 to 1 kW.

 

You switch them? That is a simple and very effective way to isolate the batteries.

Unfortuanately that means you are using an 18 Ah battery instead of 36 Ah worth of batteries at any time.

You won't get the same efficiency because you will be running at those higher currents relative to the batteries current capacity. When you fast discharge a battery you won't get as much power out of it. You will waste more energy as heat in the battery. You will shorten the battery life dramatically Things to try.

Switch back and forth between batteries every hole. Reducing their duty cycles this way will reduce their average current draw and give you back some of the lost efficiency.

I think if you could find such a thing as an automatic battery switch that would switch continuously between batteries, so each battery would only pulse current for a fraction of a second you could get back to full efficiency. The drawback is that a high current switcher could be expensive and make an awkward addition to your golf cart.

A circuit sensing the current draw of the golf cart motors and switching both batteries back into parallel when the current is a heavy load for the single smaller battery might be a good idea. The hunting bewteen the batteries is not going to be as bad as fast draining would be. Hunting is basically the batteries trying to charge each other, but because it is not 100% efficient they can end up draining each other. This is in some cases only slightly worse than the tendency of a battery to drain itself. In other cases it can be dramatic and dangerous, even explosive.

What happens under load is different. Due to internal resistances of the batteries and the load current the hunting turns into a balance where if the two batteries have the the same voltage then the one with the lower internal resistance (normally the cooler of the two) will provide most of the current. If they have the same internal resistance then it will be the one with the higher voltage that provides most of the current. There will almost always be enough differences to make this inequality a reasonable fraction of around 5%to 20% or more load difference.

But under load parallel batteries will basically tend to equalize. Assuming they are both in nearly equal -good condition - then they should be okay. Better than using one up and then switching to the other. In fact you never really want to use your batteries up. Deep discharge will shorten the life of storage cells dramatically.

 

potato pudding,

Thanks for the battery tutorial. It was really grate. Now I've learned a lot more about battery's.