I have a portable project that I am building.
I have been running off my 12v 2.5A mains power pack, and it works fine, although I think I am starting to test that 2.5A limit.
I therefore found a couple of second hand sealed lead acid batteries (3 years old), 12v 7Ah each.
I have wired them in parallel and fired them through a fuse. Fully charged, I get 12.74v at the terminals.
But, when I run my project from this battery pack (which has flashing leds x40), the leds are substantiality dimmer.
The terminal voltage doesn't appear to drop on my multimeter, so could they have lost just about all their ability to provide current?
I was trying to build this project on a budget - due to now being unemployed, so pair of new 12v 7Ah batteries is an outgoing I didn't want, especially if it doesn't fix the issue.
Is there a way to test the current providing ability of the batteries with my multimeter?
phoneystark2020:
I therefore found a couple of second hand sealed lead acid batteries (3 years old), 12v 7Ah each.
If they've been 3 years without regular charging they are utterly dead and useless. Lead acid batteries
must be regularly charged during storage. Once a month is a good idea. They should never be left other
than at 100% charge state either, otherwise they sulfate rapidly.
Problem found.
They do actually all have full charge amazingly. I got them out of some old alarm panels where they were changed as a matter of course. Not because they needed to be changed.
When pennies allow, I will get new ones.
It was a bridge rectifier I fitted inside the battery pack. I fitted it to drop the nearly 13v to down nearer 12v.
It was a 40A rated rectifier, but that appears to be the issue.
I removed it and the problem has gone away.
I therefore found a couple of second hand sealed lead acid batteries (3 years old), 12v 7Ah each.
They are not 12V, they are 13.2V. A single lead acid cell is 2.2V, 6 of them go to make a '12V' battery that isn't really 12V. If you charge them with a 12V PSU they will never be fully charged. Ideally connect them to something providing 13.2V and leave it connected, they will charge up and stay charged, and not over charge.
When I run my project from this battery pack (which has flashing leds x40), the leds are substantial dimmer.
The terminal voltage doesn't appear to drop on my multimeter, so could they have lost just about all their ability to provide current?
The question suggests that you don't understand the basics of electronics. If the voltage is holding up then the batteries are able to supply the required current. If they couldn't then the voltage would drop. I'm going to guess that you don't understand Ohm's Law, which is fundamental to all electrical circuits. Also, while not relevant to your question it would help to learn Kirchhoff's circuit laws as well, they are as important.
phoneystark2020:
Problem found.
They do actually all have full charge amazingly. I got them out of some old alarm panels where they were changed as a matter of course. Not because they needed to be changed.
You didn't say this up front! You said you found some old ones... Typical life for a lead-acid battery
in a fully charged state is several years, so they might be reasonably OK, but if they haven't ever been
cycled they might need some conditioning (controlled discharge/recharge cycles).
As for the original problem if the voltage appears not to change ,that could just be because
multimeters have very slow response, and the peak currents are actually pulling the voltage down
when its most needed - a 'scope will show this, a multimeter won't.
To test the battery just connect a spare car headlamp, these demand several amps.
Well... I did find some old ones! No idea how old they are, but I am guessing 2013 (one of them has a 13 written on it and that around when they were obtained).
No-one more surprised than me to find them still holding a charge.
Anyway, they are fine now for testing. I have a proper Lead acid charger (13.2v), so I will be using that.
No idea how old they are, but I am guessing 2013 (one of them has a 13 written on it and that around when they were obtained).
No-one more surprised than me to find them still holding a charge.
I bet they are well below their rated capacity! You could do a discharge test.
Anyway, they are fine now for testing. I have a proper Lead acid charger (13.2v), so I will be using that.