dead battery charging

[MickOpalak]

This question is about my Miata, but probably covers the Elise too.

The interior lights on the Miata were left on over night and now the battery is very, very weak. So weak that the engine won't start and the dash lights are low. If I jump start it and drive the car around for about 2 hours, will that charge the battery enough that it won't have to be replaced? BTW, the battery itself is only about 3 or 4 months old.

Thanks.

[Stan]

Originally Posted by MickOpalak

This question is about my Miata, but probably covers the Elise too. The interior lights on the Miata were left on over night and now the battery is very, very weak. So weak that the engine won't start and the dash lights are low. If I jump start it and drive the car around for about 2 hours, will that charge the battery enough that it won't have to be replaced? BTW, the battery itself is only about 3 or 4 months old.
Thanks.

Deep discharges just cream a conventional battery. You can check the state of charge by looking up the 100 % charged voltage for your battery type. If you drive for about 20-30 minutes and the battery was not too discharged this will bring it mostly back. You can check voltage by stopping the car and turning on the headlights for about 20 seconds before checking. 20-30 minutes may be enough if the car will be driven regularly and frequently after that point. A new normal style battery can only do 1 deep discharge before it is significantly affected. It might be at say 80% after that. Do that a couple times and you've got an unreliable battery.

[Buz]

That will charge the battery but the alternator is a fast charge and not as good for the battery as a slow charge. I have found that a regular car, motorcycle, jet ski battery can get away with one really deep discharge and still last fairly long. The second deep discharge usually diminishes the capacity to where it really needs to be replaced. A deep cycle battery like an Odyssey or Optima is built to take repeated deep discharges without damage.

[MickOpalak]

So I'd be better off buying a trickle charger and charging it overnight?

[Stan]

Originally Posted by MickOpalak

So I'd be better off buying a trickle charger and charging it overnight?

It's impossible to say with the stated information. You'd have to measure to see what is up. If a car is regularly driven, this will get you the last bit in another session or two. Best thing it do is not let the thing deep discharge. Or swap to a discharge tolerant battery.

[CALtd]

Doesn't anyone use hydrometers anymore? Voltage is different than potential. The problem with a quick charge vs. a trickle is the heat produced. That heat boils the battery and converts water to O2 and H2. If you monitor the battery temp and replace the water, what is wrong with a quick charge?

[JnC]

I thought stock Miata batteries were gell cells that could be deep discharged. But I don't know if they got away from this on later versions.

[EdHahn]

Agree w/ JnC - earlier Miata batteries were not conventional - dunno whether later ones are or not.

ed

[MickOpalak]

This is a replacement battery from Advanced Auto Parts, made for a wide variety of vehicles.

[TimMullen]

Original OEM Miata batteries are AGM (Absorbed Glass Matrix or something like that) batteries. They are completely sealed (although they have vents attached in case they are overcharged and "blow" gases. Miata's use these sealed batteries since they are mounted in the closed trunk of the car (like the Elise's). Some after market batteries (WestCo and ones sold specifically for the Miata) are also AGM. Some vendors (including Mazda) sell replacement wet cell batteries - then need to have vent hoses attached to vent the gases that will be produced to the outside of the car (the standard Miata vent tubes).

The advantage of the AGM batteries is the total absence of gases and any related corrosion. The disadvantage is that they don't take a charge very quickly. If the battery is drained, you can often jump start the car, but after driving it, the battery will not be charged like it would be with a regular battery (which is why Mazda uses a wet cell replacement so they don't get the normal customer complaining).

AGM batteries need to be charged at a slow rate for a long period of time. Usually around 1 amp (or less) charge for 24-48 hours after being discharged. Once recharged (at a slow rate), the battery should be back to normal. Instead of buying a trickle charger, buy a BatteryMINDer or Battery Tender Jr. and use that to charge the Miata's battery. They are designed to do the slow charge and then to monitor and maintain that charge for long periods of times. Trickle chargers simply slow chargers, but if left on for too long, will overcharge and "cook" the battery.

If the battery is an relatively recent aftermarket replacement battery that is specifically called out for the Miata, it is quite possible that it is an AGM battery. If it's a general battery, then it may or may not be AGM (probably a wet cell). In either case, make sure that it has vent tubes attached, and if it doesn't have vent fittings, replace the battery with a battery that does have them (preferably an AGM battery)...

[Fighting For Air]

Similar problem ... I think.

I tried to start my E this morning after it had sat for the last 2.5 weeks in the garage. I just get a clicking noise from the starter but no turn over.

Just a weak battery?

I'm thinking I go ahead and try a jump start and drive around for a few hours. Should that work OK for me?

I'm a mechanical idiot and have never had a battery die on me before. Any reason the battery on my E would crap out so fast after only a couple weeks?

[AlanS]

Originally Posted by MickOpalak

This question is about my Miata, but probably covers the Elise too.

The interior lights on the Miata were left on over night and now the battery is very, very weak. So weak that the engine won't start and the dash lights are low. If I jump start it and drive the car around for about 2 hours, will that charge the battery enough that it won't have to be replaced? BTW, the battery itself is only about 3 or 4 months old.

Thanks.

I did a similar thing (probably worse) to my Miata last year. I tried to open the car door when it was frozen shut with ice and snow on it. Unfortunately it opened enough to turn on the interior light and completely discharged the battery (no starter solenoid click, nothing!).

I took the battery out of the car and it appeared to be frozen solid (mind you this was in near-zero F temperature). I assumed the battery would be toast, but I figured I'd try to see if I could resurrect it. I let it warm up overnight and then began charging it (at a 2 amp rate). I suspect the battery is a little weaker than it used to be but it's still going fine. I had no problem starting the car last month in low single-digit temperatures so you might be allright Mick. Is your battery also an absorbed glass mat type? I think most Miata batteries are of this type - I couldn't even buy an aftermarket Miata battery from Sears or local auto parts stores.