Light, cheap, adequate "racecar" battery.

[90civichb]

I drive my Miata maybe 2 times a month at most. I had my alternator go bad on me at the last event and depleted the 6 year old battery to the point that I may have welded a cell together. One of my electrolyte holes is bubbling a bit, so I need a new battery. I want to go as light as possibly with staying around the same price as a normal battery would cost. So ~$175 max is my budget.

Has anyone ever used a Battery Tender on a car before? Doesn't even have a CCA rating, but does say 300 cranking amps. I'm not on a high compression motor or anything so I feel as if 300 CA should be sufficient.

http://www.summitracing.com/parts/dtn-b ... /overview/ http://www.summitracing.com/parts/dtn-b ... /overview/

[90civichb]

There is also this one at ~$80, little less cranking amps and it's a wet cell and 11.2 lbs.

http://www.summitracing.com/parts/msh-8 ... /overview/

[steverife]

My car has always used a motorcycle battery from either Walmart/Batteries Plus. Cheap, super lightweight, and easily replaceable if you have an issue.

It requires some attention (we unhook the battery between uses, you have to let the car run a bit whenever you start it, etc).

When I kept the car at my place, I'd pull the battery from the car and keep it on a tender. Once I replaced that battery, we found it wasn't really necessary.

[90civichb]

How long do your batteries last, and were they wet cell batteries? The ones from Battery Tender are AGM and like 2 pounds, hard to beat for $150 bucks if it lasts me 3 years.

[steverife]

Probably 2-3 years. I love the idea of a 2 lb battery, but haven't researched them. I know there is one kind of lithium battery that seems susceptible to fire and explosion.

I'm sure Lucas could tell you what you want and what you want to avoid, regarding that.

[90civichb]

I haven't read anything about them exploding, but I read they are very susceptible to heat. I may pick up a cheap tender from Harbor Freight for longevity. That weight is just redonkulous.

[thrdeye]

Hawker Odyssey PC 535. It ran the Subie with no probs.

[GaryM]

We've used a $20 lawn mower battery in the lemons car for 2 years now

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[90civichb]

That's a good price on that battery considering its performance. I'll keep that one on the list of choices.

[90civichb]

Which mower battery are you all using? Has the battery ever failed on you? I feel like that car is running just bare essentials, not sure if you have lights and whatnot on the car still.

[GaryM]

Standard mower battery you find at a parts store. No radio, but we have lots of lights. Never given us an issue. We sat with running lights on at a parade last week for 3 hours and it started afterwards.

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[90civichb]

That sounds like my kinda purchase then. Thanks!

[lcoleman]

Cranking won't really be the issue, it's the reserve capacity. 2 lb batteries generally don't have enough juice to hold up with accessories for very long, and they really take a beating from the alternator if they get run low and then recharged with the alternator (high current = heat). Some people get away with it, but I think it'll always have a really reduced lifespan. You also run the risk of leaving your lights/accessories on and ruining the battery.

The stock Westco battery is really good and not that heavy at ~30 lbs. O'Reilly's supposedly sells some kind of PC545/PC680 equivalent (ask for an AGM powersports battery) and I think they're pretty cheap. No experience with them personally. I'd aim in the 10-12 pound range. You can pretty much ignore CCA, I think just about any decent AGM will have enough power to crank a Miata.

With a lightweight AGM, I would always recommend putting it on a tender if the car is going to sit for ~a week or more. It actually has positive benefit to the battery, as it slowly desulphates the plates. You can leave the battery in the car when you put it on the tender.

[90civichb]

Cranking won't really be the issue, it's the reserve capacity. 2 lb batteries generally don't have enough juice to hold up with accessories for very long, and they really take a beating from the alternator if they get run low and then recharged with the alternator (high current = heat). Some people get away with it, but I think it'll always have a really reduced lifespan. You also run the risk of leaving your lights/accessories on and ruining the battery.

The stock Westco battery is really good and not that heavy at ~30 lbs. O'Reilly's supposedly sells some kind of PC545/PC680 equivalent (ask for an AGM powersports battery) and I think they're pretty cheap. No experience with them personally. I'd aim in the 10-12 pound range. You can pretty much ignore CCA, I think just about any decent AGM will have enough power to crank a Miata.

With a lightweight AGM, I would always recommend putting it on a tender if the car is going to sit for ~a week or more. It actually has positive benefit to the battery, as it slowly desulphates the plates. You can leave the battery in the car when you put it on the tender.

So from what I read about the "Battery Tender" Lithium-iron phosphate is that they do not need to desulfur so you have to get a special tender for them. This leads to a longer lifespan of the battery as well, or so the literature says.

How is reserve capacity measured? This one says it has an amp hour capacity of 14, but I am not sure what the parameters for that test were, so they seem to be arbitrary numbers in that regard. With that said, I will have a tender for whatever battery I get, so what would you recommend as a good compromise in price/weight/lifespan? This isn't a daily driver and is honestly only used once, maybe twice, a month.

[lcoleman]

Use a lithium tender with a lithium battery, and a regular tender with an AGM. The lithium tenders just have a lower voltage setpoint because lithium batteries don't like to be floated at 100% SOC like lead-acids do. Lithium batteries don't have lead plates, and don't experience sulphation. They last best at about 40% state of charge and at temperatures near freezing, interestingly enough.

The BCI industry standard RC test is 25A down to 10V or 10.5V, if I recall correctly. RC will always be measured in minutes. Some mfg'rs will use 20A or a different voltage cutoff to pad their numbers. 14Ah (a more reasonable measurement of capacity) is about in line with what you'll want. CCA numbers are measured completely arbitrarily and will not cross over from lithium to AGM. My best advice is to ignore the CCA ratings entirely.

LiFePO4 is the only lithium chemistry I have experience with, and is widely considered to be the best one for automotive starting. You won't get into a quality lithium for under $300 at a bare minimum, and I'd recommend spending that money elsewhere, like tires or shocks. The raw cells vary widely in price and the quality and longevity of that technology just isn't there yet. That one you linked will crank the motor with ease, but will really struggle handling the alternator current if it's at anything less than a full charge when you start the car (and if it's always fully charged, this will also reduce longevity). 14Ah at even a 2C charge rate, which is more than I'd be comfortable with, is only 28A. The Miata alternator is somewhere in the neighborhood of 90A capable, although in practice it will always be less.

I'd get something like this: http://www.summitracing.com/parts/xsp-s545
and the regular tender.

You can also piece together your own quality lithium if you really want, but it'll be expensive and cantankerous. This would be using 8 or 12 of these.