View Full Version : sorta OT: SLA battery chargers

20-06-2004, 09:42 PM
i'm trying to find a sealed lead acid battery charger that can handle all sorts of size batteries (12volt, 7Ah-20Ah).

nearest thing so far is this (http://www.tesa.co.nz/cgi-bin/cgimain.exe?FORMNAME=PRODCAT&CODE=LA612B) which is a variable 60 to 1300mA CHARGER.
anyone know where i could buy one? (tho prefer a 300-2000mA charger)

or who to mod a standard 2amp sla charger and add adjustable current limiting ??

20-06-2004, 10:47 PM
> or who to mod a standard 2amp sla charger and add
> adjustable current limiting ??

Well, if all else fails and you have to "roll your own" the LM 317 Application Note (http://www.national.com/an/LB/LB-35.pdf) may
prove helpful. I've used them a few times and no complaints so far.

Terry Porritt
21-06-2004, 10:36 AM
Additionally, there are some useful circuits on Bowdens Hobby Circuits (http://ourworld.compuserve.com/homepages/Bill_Bowden/homepage.htm#menu) site, including a constant current charger circuit that I modified a bit to suit my needs.

Graham L
21-06-2004, 03:55 PM
Basically, you need a constant voltage charger. I have had a lot of success with a regulated 13.8V 7A supply, adjusted up to 14.5V.

Ideally, there should be current limit until the battery reaches about 13.8 V, (when the battery has about 80% charge) then a voltage limit at 14.5 for another couple of hours to get the rest. After that, a trickle charge at no more than 13.8 won't do any harm. Keeping the voltage at 14+ on a charged battery will boil out the electrolyte.

A battery used on float should just have a trickle charger voltage limited at 13.8V. If you don't need fast charging, that would be fine too.

Have a look at the appropriate sections of the Battery University (http://www.batteryuniversity.com/).

Jaycar data/information section on their CD has an article about SLA charging. (It's probably on their web site too).

Peter H
21-06-2004, 04:48 PM
What I have used for years, is a Dick Smith Kit K3127, fitted to an Arlec 4amp standard charger. Use it on a 32ampere/hour 12v for an electric golf trundler.

21-06-2004, 05:28 PM
I built and use the same kit, which reduces the current to a maintenance trickle at a preset full charge voltage (typically around 14v)
However it will deliver the full current until then (up to 4A, depending on the charger). This would be way too high for a 7AH SLA and would produce excessive gassing, risking rupture.
I use it with a 15v plugpack for small SLAs, larger charger for the car.
It would pay to research max charging currents for small SLAs to be safe and avoid damage.

21-06-2004, 05:38 PM
thx all :)

i have a 4A automatic charger (switches to trickle when bat is full) for the car batteries but i was kinda hopeful to get/make one that is current adjustable so you can limit the charging current for all the different batteries. thus saving the need to have 4 different size chargers. the lm317 notes look interesting :) i will have a look at the DS battery charger circuts and see if it can be modded to suit.

21-06-2004, 08:04 PM
Sounds like your charger has a similar circuit already (DS one is from Electronics Australia) - based on an SCR to switch the current on/off, pulsed by a 555 chip. Not true current limiting.
Suppose a cheap/crude/inefficient (who cares) solution for current limiting would be to use monster resistors (calculate resistance and wattage ratings first) - just a thought.