I have recently become the proud owner of an electric start lawn mower, but I've never cared for an SLA battery before. This machine has a 2.8 AH 12V battery. That's plenty of amp-hours for a 3 to 6 second starting burst. I suspect that size battery was provided because anything smaller would have too much internal resistance to fire up the starter motor. Being a lawn mower, it will spend a lot of time not being used, even months in the winter. I have no trouble making a float charger, but I wonder,

Is it best to float charge the battery all the time, sometimes for months at a time with no drain, or let it go down 10% or 20% between charges?

Next question, The label says to float it at 13.6 to some higher number that I didn't pay attention to. Should I follow the instructions and use 13.6V or a tad higher? (After seeing the POS charger that came with the lawn mower, I suspect the manufacturer wants me to fry the battery so I will buy them often.)

I did a trial charge at 13.41V and the battery seemed very happy after the charge current went down to 8ma. By happy, I mean that a week after I removed the charger, the battery voltage was 13.1, above what you would expect from a car battery, I think. Kind of messes up the calibration in my head. Somebody want to help me recalibrate my thinking?

 

Those small sealed batteries are very good at retaining a charge. They have a very low self discharge rate. They can go for years without a charge if kept in a constant airconditioned atmosphere. That being said, it is not good to do this to them, and also it is not a good to keep them 'floating' at high voltage levels either.

For a prolonged duration of weeks I would recommend simply keeping it in the airconditioned comfort of your house between uses. Recharge it immediately after using and DO NOT overcharge it. If you must keep them in a floating condition, lower the float voltage to 13.2 volts after charging them.


THe higher voltage reading of 13.0 or 13.1 volts is common with this type of battery and nothing to worry about. The voltage level falls to 12.8 or 12.7 after a few months this is still not a problem. If the voltage gets below 12.6 volts then sulfation of the plates will start rapidily.

Overcharging is a common mistake(more is always better right?) and the result is a gradual loss of water from the electrolyte leading to excessive acidity and resultant damage to the lead plates.

Keep them cool (60-78 F) and fully charged when not in use and they can last many many years.

 

Lear a lot about the care and feeding of batteries at the Batrtery University.
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

 

Here's what I do with RV and mower batteries. At the end of the season (before the first snow), I take all the batteries out of everything and stick them on the floor of the garage. Then from October to April I try to remember to connect the battery charger to them every couple of months to keep them charged. When I take the lawn mower's battery out every spring and hook it up again, the lawn mower always starts (it's a riding mower and has a battery about the size of a large motorcycle's battery). If there are access ports to the cells, I make sure they're topped off with distilled water before putting them back in service.

I've been doing this for a number of decades and the local battery dealer tells me I get rather good life from my RV batteries (some have lasted us 8 years), so I must be doing something right.

 

First, the negative: Battery U does not address SLA's or Gel-cells as being any different from car batteries. No help there.

I find universal agreement that getting down to 12.6 resting volts means it is time to recharge the battery. Measuring resting voltage will not be a problem because my dinky little SLA will spend most of its life resting.

I fear my battery will never get down to 12.6 volts because of the idea from Battery U that the battery should be raised to a "charging" voltage, much higher than the float voltage, every 6 months. Something about equalizing the cells. If my battery doesn't get down to 12.6 volts in 6 months, the float voltage seems moot!

After seeing how the battery acts after a charge to 13.41 volts, I feel that a lower float voltage is in order and I thank Kermit for specifying 13.2V. If my battery goes down to 12.6 volts in the summer, when I have to mow every 6 days, I will float it during the summer. I wish to hear opinions about the charging voltage that should be done bi-annually. If you wish, I can get the battery and read the labels for you about exactly what the mfg recommends.

 

This Leoch brand Maintenance Free Sealed Lead-acid battery says,

standby use: 13.5 - 13.8 V
Cycle use: 14.4 - 15.0 V
Initial current: Less than .84 A

This seems to say, "Charge to 14.4 to 15.0 volts, every 6 months, at less than .84 amps."
and, "Float at 13.5 to 13.8 volts between charges".

1) Have I interpreted the instructions correctly?

2) I suspect that these voltages are a bit high, partly because the charger provided is limited to 100ma (C/28) so the battery will be full a long time before the charge voltage gets to 15 volts.
With 12.6 as the "charge me" point, I think floating at 13.2 will avoid sulphation while also avoiding overcharging.

Your opinion is requested now that you have more information.

ps, I calculate 28 starts to use up 10% of the battery's amp-hours. (12 amps for 3 seconds per start) That is why I fear the battery will never get down to 12.6 volts during the 6 months of dry season when I only mow every 8 to 12 weeks.

 

Always test your battery voltage under load. I have two "dummy" loads, one is a 50 watt 12 volt incandescent lamp (4 amp load), the other is three 313 bayonet bulbs in parallel (900 ma load). A properly charged SLA battery will read 11.0 to 11.5 volts under the 4 amp load. If the voltage under load is 10.5 or less, you have a paper weight containing hazardous materials.
The no-load battery voltage is nice to know, but doesn't indicate the condition of the battery.

 

That method requires that you know the appropriate load for the associated AH rating and type of battery.(starting or deep cycle)

a 4 amp load on my 220AH 8D batteries would barely register any voltage drop at all, and the same 4 amp load on the OP's 2.8 AH SLA's would swamp them