Max charge current of a 90Ah SLA

[AbyssUnderground]

Hello all,

I just a have a small question for you guys about SLA batteries.

I have a 90Ah SLA battery which I got from eBay several months ago. In the future I'm planning to add a split charge system to my car for extra charging capabilities when I go camping incase the solar can't keep up.

My concern is that the specs sheet for this battery say the max charging current should be 26A at 14.4v. Now I know that a diesel alternator is quite capable of 60+ amps at 14.5v during normal driving. Now I'd rather not limit the secondary charging system to 26A as I don't know how far or how long I'll be driving each time I do, so I want as much charge put into the battery as possible.

My question is that can I safely apply more charging current than it says in the specs? I know most people's responses will be to stick to the specs recommendations but I want to know the facts that other people have come across when charging these SLA's personally.

Thanks in advance for any responses.

Andy.

[Flux]

This is just my personal opinion, others may not agree.

With a split charging scheme you will probably have a bit of cable resistance and it will likely be higher than the main charging circuit. After a start, the main battery will likely take more than your camping battery and the current to it should not be too high. The worst case would be if your camping battery was very low then it may grab more current than the rated figure that you have, but it will not hold this current as it becomes more charged so even then I don't think there will be a problem.

If you are prepared to take some measurements then you could play with the lead resistance to limit current to about 30A. Normally at low state of charge most batteries are not too bothered by charge current but it has to be limited as the voltage rises. SLAs have some limitations that don't apply to flooded ones and in many ways they are a pain, so the manufacturers probably specify a limit for good reason but I don't think it is absolute and the average car charging circuit will not likely exceed those limits by very much or for very long.

Flux

[wlcoldiron]

Hi

Place a voltage and amp meter in the charging circuit for the battery you want to charge. Start the vehicle and let it run for a few minutes and then hook up the charging circuit to the charging system and let it charge the battery. Most automotive alt will charge less than their rated output because the alt will burn up if it charges its rated output all the time. Usually the output is about 25% of the rated output. You could put a charge controller for the battery. RV's have a battery isolatorbut I don't know if the isolator has a current limit built in. Hope this will help you.  Weldon

[ghurd]

So you want first hand experience with half-axed battery charging?

Like Flux said, I don't think you will exceed the maximum amps very long. Probably less than 20 seconds. Maybe less than 5 seconds.

I personally would not want more than 8A charging that battery for more than a few minutes (10 minutes?) either way.

The surface charge will come up to the system voltage very quickly, and the charging current will drop.

You don't say how you will charge the battery while driving.

A lighter socket jack into the lighter socket will have plenty of voltage drop, if the wire from the lighter to the battery is marginal, say 16 gauge and 6~8 feet long.  The voltage drop with a low battery will limit the amps at that time, but as the SLA reaches a higher voltage the current and voltage drop will be less, allowing the battery to reach the system voltage in a semi-controlled fashion.

I would recommend a couple things...

Like `car charging' the battery every morning for a few minutes if the battery is below ~12.35V or if the battery will be heavily used that day.  Get it up past 14V for a couple minutes, then let the solar finish it.

And lots of spare fuses for the lighter fuse.  Just in case.

And don't use `genuine store-bought jumper cables' from battery to battery.  Something like 8' of #16 would be OK, and they could be a good way to get the SLA up near system voltage before using the lighter plug, if the lighter plug is blowing fuses.

And don't let the SLA battery get warm while charging.  Same for the wires.

Hope it makes sense,

G-

[AbyssUnderground]

I was planning on having a proper split charge system having cables running to the boot where I could put them on there (mentioned in the first post). Why no higher than 8A? Its a big 90Ah SLA battery with a rating of 26A charging current so how will 8A be dangerous?

[ghurd]

The 8A thing is for gassing, best guess.  

Some of the 7 to 20 AH SLAs I have used were shot after charging at C/10 or C/8 at the peak.  Just a few times too.

Some of the 7AH are abused but only get 220ma max charging, and they are fine after several years.

One 7AH is the most abused (by the standard of being left in a low SOC) and 4 to 6 years old.  It started at 12.4V, put 5A into an inverter for 40 minutes before LVD, and seems to be properly charging now at 150ma with a VW PV.  It was never fast charged and I believe that is why it still lives.  Others were not so abused with standing undervoltage, but abused with long fast charging, and they were long ago recycled.

Camping is a different animal. Take what you can get when you can get it.

G-