Battery question

Marke (Mark) · #1 09-05-2010, 02:05 PM

I am going to add a second battery to my battery box for use over a couple
nights. Am I correct in assuming I just connect them in parallel and charge
as I would normally ?

tlgerdes (Trevor) · #2 09-05-2010, 02:27 PM

In essence, yes. Parallel batteries give you a longer output, series batteries give you a higher output.

The issue of charging though can vary depending on the age and type of battery. One possibility is if you have one old and one new battery, one could charge faster than the other and damage could ensue by over charging. Unlikely, but possible.

Marke (Mark) · #3 09-05-2010, 03:17 PM

Great thanks thats what I thought , I am using 2 new AGM the first one
was much smaller in size than I thought so can easily fit 2 in the battery
box to give me 66Ah .

pjphilli (Peter) · #4 09-05-2010, 05:38 PM

Hi Marke
I have found that it is sometimes problematic running two rechargeable batteries in parallel. If one is older than the other (ie lower voltage) then the newer one will start discharging into the older one. Probably better to use the batteries separately and change over when the first one is discharged. Alternatively, you can isolate the batteries by charging in parallel through a suitably rated diodes (eg rated at say twice the anticipated maximum current and heat sink if necessary) connected between the charger and each battery. This isolates one battery from the other. You could similarly isolate the batteries by feeding your equipment likewise through diodes, however, you would lose about 1/2 volt through the diodes and the stronger battery would feed first until its voltage reached that of the weaker battery, then they would both begin feeding. The better solution is as above ie switch them over but of course making sure of the correct polarities in the dark! I would suggest rigging them up on a single pole double throw switch so that changeover could be done quickly and safely. The problem with switching over of course is that you may have to restart your gear.
Cheers Peter

Barrykgerdes · #5 10-05-2010, 07:58 AM

Paralleling the batteries is not a good idea although it can work if the batteries are truly identical. Switching is a better idea. A short period of parallel batteries is used in jump starting cars but this never lasts longer than a minute or so.

I have experimented with some older 12v 20AH batteries from my golf buggy. I made two banks of two batteries and charged them up. It worked quite well for two or three re-charges but after that both sets of batteries are virtually useless. Won't charge any more. The reason most likely being the continued leakage from the better battery into the poorer one over a period of time.

Barry

Marke (Mark) · #6 10-05-2010, 11:21 AM

I may have to give this more thought now , I am using 2 identical new
AGMs but maybe a safer option to just switch them over when needed.
I could charge them both individually then just parrallel for the weekend
and save on any mucking around while away.