Best way to keep backup SLA batteries topped up in a vehicle?

[bigal_scorpio]

Hi to all and seasons greetings,

I am pondering the idea of building in a set of maybe half a dozen SLA batteries that could give the regular car battery a boost if it became flat (happened to me the other day) and was wondering the best way to keep them charged without damaging them so they would always be ready at the flick of a switch.

There seem to be many opinions on this subject on the net, many conflicting, so I thought why not start a discussion on the subject and maybe get down to some real answers.

I have quite a few spare SLAs knocking around 12v 7Ah and was thinking maybe 6 in parallel which should be plenty to pull a dead battery back up, after all the jump start packs you can buy only contain a single SLA, usually 7Ah!

Any thoughts on the matter guys?

Al

 

[3v0]

Make a case for the SLA's and keep them in the turn hooked to a simple car powered float charger. Pull em out to start the car when needed.

I would rather have smart monitoring on the existing battery to let me know of its condition and possibly disconnect it if needed to keep it from going flat while the car is not in use. Let me know how much current was used when the car is off. That sort of thing.

 

[Sceadwian]

This is why they invented jumper boxes, buy one, if the jumper box you're buying has a standard SLA battery in it, it's cheap buy a better one =P. A good jump box uses special high surface area batteries (at least the compact ones) so that it can provide huge cranking amps and is only mean to be used occasionally and then topped off the rest of the time, just like car batteries, only their duty cycle is lower and they have to be recharged immediately after they're used or they degrade quickly.. If you're in a pinch and you have a charged SLA handy you can use a power resistor in parallel to leak some current back into the FLA back for a few minutes to give it enough to turn the engine over, you could just straight wire the SLA and the FLA in parallel and let the FLA drain as much juice as it can, but the wires might get hot and the SLA would probably be shot afterward, I'd do that in a pinch.

My primary thought is don't let this situation occur. Keep your batteries maintained. Car batteries are meant for starting the car ONLY and buffer the power from the alternator during run time, they are not designed for any other purpose, if they're left alone they last 5+ years, it's when you get to fiddling with them and doing things like running a lot of accessories when the vehicle isn't moving for long periods of time that will kill one. If you do something every now and then that causes the battery to drain down too low to start, well trying to add another system that you have to remember to keep charged is not gonna help things =) Especially considering any battery that this happens to on a regular basis is gonna be unreliable for cold starts after the first year...

I've seen lighter socket 'jump' start kits that are nothing more than a pair of lighter sockets on a long wire with a resistor valued to limit the current to the wiring/lighter socket current. They work fine, you plug one end in to a dead car, the other end in to a running one and wait a bit done deal.