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I need to store/conserve a sealed lead-acid battery (Bosch S4) for 9 months. There are two options I am considering:

  1. Leave battery disconnected and recharge before use.
  2. Charge with solar panel during the 9 months.

With the first option I expect a self-discharge of about 50% overthe nine months with associated crystal formation, etc.

With the second option I fear overcharging. Typically solar chargers restart a timed charging cycle with every sunrise: e.g. 2 hours boost/absorption before falling back to float voltage for the rest of the day. - Independent of whether the battery was discharged at all.

Which option is likely to be best for sealed lead-acid batteries and why?

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Neither approach is good.

See http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.

Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell.

I believe the main issue, relevant to this question, is sulfation

Solar cells and wind turbines do not always provide sufficient charge, and lead acid banks succumb to sulfation.

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During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline that deposits on the negative plates. This leads to the development of large crystals, which reduce the battery’s active material that is responsible for high capacity and low resistance. Sulfation also lowers charge acceptance.

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  • \$\begingroup\$ Thanks for the interesting quotes. The question remains which of my two options is less bad. \$\endgroup\$ – ARF Oct 13 '13 at 18:01
  • \$\begingroup\$ @ArikRaffaelFunke - As the RedBrick has written they are both bad. If you had to pick one select the solar option but make sure to evaluate requirements well enough so that the solar cells can provide enough energy to support the full change of the battery. Also make sure that the charger circuit supports operation like a float charger does so that the solar cells do not overcharge the battery when a nice sunny spell comes along. \$\endgroup\$ – Michael Karas Oct 13 '13 at 18:22
  • \$\begingroup\$ A good solar charge controller will not allow your battery to overcharge. A better solar charge controller will allow you to adjust the boost and float voltages to match your battery chemistry and operating temperature. There is no reason to expect overcharge or undercharge while connected to a decent solar charge system. \$\endgroup\$ – david Oct 14 '13 at 3:33
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I have recently had reason to spend a lot of time and effort analyzing one German made controller. Monitoring current/temperature/voltage over days and weeks I found that it does not kick off a boost cycle unless the battery voltage has dropped below 12.6V. This is exactly what the manual said would happen, but I had my doubts. I also have another, cheap eBay MPPT solar controller that runs at 13.8V and does not have "smart" charger capability.

Unfortunately, the manufacturers of solar controllers rarely publish there battery charging regimes, so unless you do what I did, and monitor outputs over time, its impossible to know what a particular controller will really do.

As you only need to keep them topped up, a small solar panel (typically go up to 5% of the AH capacity in solar panel Watts - so 5W solar panel on a 100AH battery) without a controller can be done as well. With a diode, this means no night time current draw, so no micro charge/discharge cycles. A controller will use 10-20mA so you need a daily input of 1/2AH to recover those loses.

Letting the batteries go down to 50% for such a long time will do way more damage than a small over charge. Install a 2-5 Watt (per battery) Solar panel though a blocking diode, or a 5-10 Watt panel with a controller.

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