24v charging from a 12v source problem

hi i've come across a problem ,im trying to charge 2x12v,75ah batteries wired in series=24v/75ah from a 24v/4ah charger pluged into a 1500w mod sine inverter ,connected to 2x12v 70ah batteries in parallel =12v/140ah the problem is its not charging ,the charger says it is and finishes charge (usually 2hrs) but its not charging ,i know this because when i plug charger into wall socket it charges again for 2hrs. only thing ive noticed is when i plug in charger to inverter load comes on and fan starts but after 10min fan stops but load stays on ...any help would be great thanks

What do you mean by a 24V/4Ah cCan you garger?
Get an ammeter in the charge circuit?

The charger is most likely terminating because either the voltage or current have met certain values.
As mr skier says an ammeter is a good idea, and a voltmeter too if you have them.
Do you know for sure the batteries your charging are actually discharged?

thks for reply ,i think im onto it ,the batts im charging from may be low on amps ,they solar charging from a 40w/1.5a panel ,ive read they need 10% of ah to be able to do it ...can batts be volts correct but no amps ?

btw charger doing its thing volt wise ...im thinking inverter running out of amp supply ie im trying to "invent" amps

Battery voltage can be only slightly different from discharged to charged, for lead acid 'car' type batteries empty is around 12.2v and full 12.8v, or thereabouts for the critics.

they are deepcycle seald batts

dr pepper said:

Battery voltage can be only slightly different from discharged to charged, for lead acid 'car' type batteries empty is around 12.2v and full 12.8v, or thereabouts for the critics.

Click to expand...

Possibly after standing for some time - but the "On charge" voltage would be between 13.8 - 15V at full charge, depending on the charger.

The "two hours" each time is suspicious. I'd try a different, higher rated charger.

Yes thats correct, I read somewhere 20 mins after charger disconnected.

float charge is 13.8v (writen on batt) and you correct straight after charge they read 14.1v but after aprox 30min they drop back down to float and stay there,that time changes alot depending on temp.and yes an amp meter would be nice but are expencive to read that many amps (aprox 700amps) but i do believe its the solar panel not delivering enuff amps,what i read said you need 10% amps of the ah so for 2 x12vx70ah in parellel =12vx140ah so i need a min of 14amps coming from solar panel to properly charge them and im currently only giving them 1.5amps

It does sound like your charge system is just not getting enough power.

Lead acid batteries are not all that efficient when being charged and if the charge current is not high enough to fully charge them in 24 hours or so, you are likely to have problems.

i guess what im asking is can a batt have correct volts but no amps ,and does the 10% rule per AH apply/true ?

Kinda, I'm not a pro on batteries however as a battery discharges it internal resistance goes up, so the voltage no load compared to loaded changes, ie a charged battery may read 12v off load and 11.9v loaded, whereas the same battery discharged might show close to 12v off load and only 11v when loaded.
If you want a 'fuel' guage for your batteries you can get such things off ebay fairly cheap, they measure the charge & discharge energy levels & give you an idea of the battery capacity, once you've programmed it with the battery capacity & technology.

i once had a strange problem with the battery in my truck. the headlights would come on very bright, but the starter didn't even click when the key was turned. it turned out the battery had a cell that had high resistance..

talking of strange problems i once had a car that would start and run but if i used indicators lights etc they would blow fuses ,turned out batt had randomly changed polarity .....5 days looking for shorts in wiring ,another 6 days putting car back together....and yes alot of excess screws left over ..................

so with your strange problem you have answerd my question ,they can have enuff volts but no crank amps ..

Back when I was still in the TV trade, pretty well every colour portable we sold that worked on 12V would be brought back, complaining it doesn't work.

In almost all cases, the customer was using an old battery that they removed from the car because it would start the car any more!! - if it's not good enough for a car, it's not good enough for a colour TV.

In one case, that wasn't the problem - the customer lived in a 'small' manor house, and while he was having extensive restorations done he bought a luxury caravan to live in, so he was on-site. As I recall, he paid £30,000 for the caravan, and it was a LONG time ago - so it was a LOT of money.

So like others, he bought a colour TV that would work off 12V, and then complained it wouldn't work - brand new caravan, brand new battery - brought the TV back, and I checked it on 12V, working perfectly. So it was arranged I would go out and have a look.

So I connected the TV in his caravan, turned it ON - and the LED briefly lit up, then went out.

As I was pre-warned of the problem, I had taken a few 'bit's and pieces' with me - two meters (one for amps, one for volts), a car headlight bulb (for a dummy load), and a couple of long pieces of wire.

First I went to the battery, it was in a 'battery box' on the outside of the caravan, I unlocked that, and lifted the battery out - first problem, the wire to the battery was only thin 5A twin cable, like a TV mains lead, and there was like 20 feet of it coiled up in the battery box. The thin wires then went to a fuse and distribution board inside, and to a number of 12V sockets.

I inserted my ammeter in the positive of the battery, and connected one of my extension wires to the output of the ammeter, and ran it through to inside the caravan - I then used my dummy load on the 12V socket, and used the extension lead and voltmeter to measure the voltage drop across the +ve wire. I then repeated the same test on the -ve wire - applied a little ohms law with the volt meter and ammeter readings, to calculate the resistance of the positive and negative feeds.

I can't remember the actual values now (it was decades ago), but I seem to recall it as losing something like 3 or 4 volts from battery to socket with about a 5A load. Last I heard he was on the phone playing hell at the caravan agent, as it was really pretty crap for a seriously expensive luxury caravan.

Since then the customer has died, and the house was sold (for something like £1,500,000 probably ten years ago), and among the details on the sales literature was an 'indoor shooting range' - which was interesting for the UK.