SLA chargers

TheBeam · Sep 22, 2004

I'm looking at getting one of these . I'm no electrician, so I need someone to explain to me, how does this charger know when to shut off? Actually it doesn't shut off, or go into a trickle mode, it "floats". These say nothing about using -Delta v.

Also, how does it compare to this Would I be better off getting this one instead of seperate chargers for 6v,12v etc?

TheBeam · Sep 22, 2004

How do these know when to quit charging?

Anybody? /ubbthreads/images/graemlins/confused.gif

jayflash · Sep 22, 2004

I believe they are voltage controlled. When they reach a preset voltage (say 13.6v for example) the current output is reduced to the trickle rate.

mattheww50 · Sep 22, 2004

Actually for most lead acid cells it is usually a combination of a linear unregulated voltage supply with a known maximum voltage, and resistance in the charging leads. This makes them simple and cheap. SLA chargers are probably a bit more careful, and probably do incorporate voltage sensing and a switching power supply. Standard Lead Acid battery chargers are just an unregulated linear supply and some resistance in the charging leads.

The voltage is higher than the fully charged voltage of the battery. The resistance in the charging leads and the loading of the power supply limits the initial charging current when you connect up a truly 'dead battery' to whatever the charger is rated for. The combination of the lead resistance and the load in the supply limits the current. As the load comes down, the voltage from the rises.

So for instances a 10 amp charger might have a 16V supply, and about 1.5 ohm in the charging leads. When the battery is fully charged, you have 2.8 volts across 1.5 ohms, so you end up with a trickle charge just over 1.8 amp which is fairly typical.

RussH · Sep 23, 2004

The first charger mentioned charges at 500ma (current limited)until the battery gets to 15v (slightly overcharged, IMO), at which point it switches to 13.8v constant voltage. Most manufacturers recommend a "float" or continuous charging voltage of 13.4-13.9v. This sounds pretty good to me for an under $10 charger. I've been using LM7815 chips ADDED to wall warts to do the same thing. I may just buy a couple of these myself.

The battery minder has a PWM (pulse width modulated) charger and pulses the battery at a high voltage which apparently removes sulfation and renews battery capacity. One advantage of these is that they can be used at various voltages such as 6v-12v (some cover 6v-48v or more) with no additional parts. The width of the pulse is simply varied for the proper charging rate at the load voltage.

The method mattheww50 outlines is common in automobile battery chargers and is their justification for claiming a 'trickle' charge capability. It leaves a lot to be desired, IMO.

eluminator · Sep 23, 2004

I've got three of the batterymart ones. (the "these" link in the first post) They seem to work fine, although there was a bad solder joint in one that caused the green LED to light only intermittently.

When charging a battery that has been discharged, they put the maximum current into the battery initially. That is, .5 amp for the .5 amp charger. As the battery voltage rises, the charge current diminishes. When the battery voltage reaches about 14.8 volts (or 7.4 volts for a 6 volt battery and charger) the charge current is approximately half the initial current. At this point it switches to float charge. In float it supplies around 13.3 volts (or half that for a 6 volt system). At least that's the way mine works.

I think there are a couple of pots inside, for those people that can't stop tinkering until they break something. /ubbthreads/images/graemlins/smile.gif

Presumably if you leave it for a few months, the battery will self-discharge enough for the charger to switch back to fast charge.

I've tried to trick them by hooking them up to a newly charged battery, but they figure it out within 30 seconds and switch to float. The NiMH chargers can't figure it out so fast.

I've also used one to top off a car battery that had been sitting around for a couple of months. It worked fine.

These chargers use a simpler algorithm than any I've read about, but I think they work fine.

You guys that are in such a mad rush to charge your batteries that you can't live without a 15 minute NiMH charger might want the 1 amp charger. Just check the label on the battery to see what the maximum permissible charge current is.