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I have 2 12V SLA batteries for recreation use in my camper. One very old (+10 years), another a bit less (unknown, maybe 5 years?).
I noticed both of them were working fine in the beginning, but now the first one is completely used up and ready to recycle (5V after 5H charge!). But the second one is also more or less broken. For example it quickly discharge to 9V after half of the charging time on 60W only!

Now, unless the first one was its due time and because I left it too long connected parallel to the second one (can that be?), I suspect that this DC-DC converter I use for my laptop is at cause. The DC-DC converter 95W and converts from 12V to 19.9V, but it pulls amps in a very alternate way. E.g. alternating between minimum 4A and maximum 9A.

So, I'm wondering if this alternating current can quickly wear out an SLA or other negative effects?

But my more general question remains priority: Can a (certain designed) DC-DC converter destroy or very quickly wear out an SLA?
With very quickly I mean, in months or so, compared to the years left in it. (Since I'm going to buy a AGM battery as replacement and don't want to kill it.)

Thank you, more experienced battery users!

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    \$\begingroup\$ A better term than "alternating" might "pulsating". \$\endgroup\$
    – Wesley Lee
    Jan 29, 2017 at 18:45
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    \$\begingroup\$ I would consider the 10 year old battery beyond end-of-life (probably with a few shorted cells), and the 5 year one probably close to it. I wouldn't blame their poor performance on your DC-DC converter. \$\endgroup\$
    – Peter Bennett
    Jan 29, 2017 at 18:47
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    \$\begingroup\$ If you connect them in parallel, the weaker battery acts as a load on the good battery and discharges both. Replace both batteries at the same time. The load is NOT the problem. \$\endgroup\$
    – StainlessSteelRat
    Jan 30, 2017 at 0:17
  • \$\begingroup\$ Are you guys saying that DC-DC converters "can not" be the problem or just that the other factors are more likely? Because I need to know for my new AGM battery that I'm going to buy that DC-DC converters cannot be a problem (if used normally and not below 11V e.g.). \$\endgroup\$
    – e-motiv
    Jan 31, 2017 at 19:25
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    \$\begingroup\$ Lead acid batteries can be pre-maturely destroyed by over-charge or over-discharge. But generally not by discharging with pulsating current. \$\endgroup\$
    – user57037
    Feb 1, 2017 at 0:27

4 Answers 4

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The switching nature of a boost regulator is typically not a problem for most batteries. The concern here is the current pulled. If it is in the acceptable range of your battery. Or if you let the battery drain too much. Trying to pull AMPS from a dead battery will hurt it. Even deep discharge batteries would be affected by this.

But also, those batteries are very old and you are mismatching them. If you have them in parallel, one will attempt to charge from the other due to their different conditions. So replace both with new and identical ones.

Additional concerns, continuous load current of a battery is not the peak, surge or pulse current. The Datasheet for your battery will have both, as well as the expected capacity at any given load. A car battery may have a 600 CCA rating, but this is over seconds and not minutes let alone hours.

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  • \$\begingroup\$ I think your answer is the most complete one, including the direct answer to my question. But can you confirm that with "typically" you mean that if used normally and not below 11V e.g., DC-DC convertors are not at all known to have any negative effect on SLA's? (To assure myself so I don't destroy my new AGM battery that I am about to buy.) And 9 amps for a battery that can give 200 amps is not the amp drain, you mean, right? Or is there another lower long-term maximum amp drain? Thanks! (I would appreciate if you'd put that in your answer too for other people maybe..) \$\endgroup\$
    – e-motiv
    Jan 31, 2017 at 19:29
  • \$\begingroup\$ If your SLA can do that then that's fine. There are smaller ones, like for Uninterrupted Power Supplies. Check your battery's datasheet to see if a constant 9 Amps is okay, versus a peak or surge draw. A car battery may have 1000 cold cranking amps but that's over a few seconds. It does not mean it can provide 1000 CCA continuously. And yes, typically is as long as you don't deep discharge it. A battery tender or cut off circuit may be a good idea. \$\endgroup\$
    – Passerby
    Jan 31, 2017 at 23:59
  • \$\begingroup\$ Thanks! I don't have the sheet, nor can I find it back on internet. What would be the typical maximum constant draw for a 99Ah car battery? There must be a general rule for this, no? Like 10% of its cold cranking or its capacity or so.. ? (To be clear, the 200 I meant, was the cold cranking, but I might have forgotten; it's probably more near your 1000A.) We're not talking about the longest time, right (like normal draw untill 11V)? \$\endgroup\$
    – e-motiv
    Feb 1, 2017 at 16:07
  • \$\begingroup\$ @e-motiv manufacturer and part number? \$\endgroup\$
    – Passerby
    Feb 1, 2017 at 16:32
  • \$\begingroup\$ Here are some things on my battery that say something. Dunno about part number or manuf. There is a side that I can't reach yet. Can check in a week or two. --- 58827 (Japan Quality) - 88Ah - 395A(DIN) - Maintenance free --- There is no general rule? --- Thanks \$\endgroup\$
    – e-motiv
    Feb 3, 2017 at 20:30
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A bigger factor is if you let the DC-DC converter draw the battery down until it is completely flat- that will kill the battery in short order.

It will do that even if you have nothing connected to the output of the DC-DC converter.

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If you want long life from an SLA, don't routinely discharge more than 50% of its rated capacity. The second 50% is for emergencies only. If you always take it down to 11v, then you'll only get a few hundred cycles

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Ordinary SLAs are done often after three years of continuous use in 24/7 UPS power supplies.The 10 year models are very expensive. They can survive that long only if the charge-discharge cycle specifications are followed excactly. As already commented, Your load is not a problem, but if you let it draw the battery flat, it probably stays flat finally because some of the cells start to get reverse current. The "cabin" hints long pauses to exist between the periods of usage. Let your batteries stay unused as empty for months, they will wake up much more tired.

Check the available quidance. An example:http://www.powerstream.com/SLA.htm

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