Battery charging question

SystemSystem Posts: 2,511 admin
Hi folks,
Just getting into solar, battery banks, etc. and had a quick question for a mind much wiser than mine.
I've done quite a bit of reading re: battery charging, not taking them down below 50%, etc. I noticed that 12.1 volts is about the 50% battery level on my 12 volt- 2 battery bank. Are the numbers I should refer to on my volt meter while there is no load (at rest) or under a load? I also note that my B&D 750 Watt inverter sounds the alarm at around 11.2V and will shut off shortly after. Is this level OK to use to maintain battery life long term? Seem to me this would be way too low!
I appreciate your comments. I'm looking forward to getting into solar/batteries and with everything going on in the world (weather, wars, "rumors of wars" - if you catch my drift), I think its a good time to move in this direction.
Thanks again.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery charging question

    Welcome to the forum.

    I guess you've read through the battery FAQs http://www.windsun.com/Batteries/Battery_FAQ.htm and seen the Voltage chart. Those numbers are for "at rest" batteries; no charge going in or loads going out and preferably after sitting for at least 3 hours. Not the best way to tell SOC. Checking the Specific Gravity with a hydrometer is better, although also more accurate with "at rest" batteries (bubbles can cling to the float and make it more buoyant). You can also use a battery monitor http://www.solar-electric.com/metersmonitors.html to keep track of the ins and outs and relative SOC. No system is perfect.

    Shutting down the inverter at 11.2 Volts is good. Most 12 Volt inverters will allow you to draw the batteries down to 10.5 which is essentially dead. Too much of that kind of activity and you wind up with scrap metal.

    We try to avoid political discussions around here, btw. ;)
  • System2System2 Posts: 6,290 admin
    Re: Battery charging question

    Thanks for the advice and yes I did read the battery FAQ's but must have missed the "at rest" part. So just to clarify the inverter alarm...I understand the importance of it going off at 11.2 under a load, but woudn't that mean my battery is probably way to far down at that point (below 10% SOC). In other words, if I hear the alarm go off I've run the battery too low by then.
    Thanks again.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery charging question

    No; 11.2 Volts under load would be higher without the load. So the battery will be okay there. Normally you'd want to keep an eye on it and get a good "feel" for how much power use is going on, then cut back loads to keep it from going too low. The low Voltage shut-down is there to prevent damage to the battery; it is not an indicator that you've used "X%" of your available power.
  • System2System2 Posts: 6,290 admin
    Re: Battery charging question

    Got it!
    Thanks again.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery charging question

    i'll add that how far the voltage drops will coincide with how big of a load is on the battery. that means if it has a light load that it could be drawn too far for soc purposes before it cuts out.
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