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Old 27-06-2012, 05:50   #1
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Batterys again

Sorry I know this topic has been brought up many times but.......... My house 3 x 100amp battery's get down to 12.4 volt I start to worry. Can I bring them down to say 12 volt safely ? And then to charge them back full at 20 amp charge rate it will take 15 hours to charge ? Yes/no?
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Old 27-06-2012, 06:42   #2
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Re: Batterys again

Depends on what you mean by "safely." Deep cycle, lead-acid batteries will last longer if they are not routinely discharged too deeply. Generally speaking, the lower the average depth of discharge, the longer the batteries will last. So routinely discharging them more deeply may somewhat shorten their life.

On the other hand, on a dollars-per-watt basis, you may get more out of the batteries over their lifetime by discharging them somewhat more deeply than you have been. This is because it takes less time (and therefore usually costs less) to recharge from 50% to 85% or so, than it does to recharge from 85% to 100%. Figuring all those costs out, over the lifetime of the batteries, can be a fairly complicated calculation.

In any case, there is no immediate danger to discharging them to 12 volts. It's not like you are going to kill the batteries straight away by doing that. Lots of people routinely discharge their batteries to 12 volts (or even less) before recharging them, and still get many years of use out of them.
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Old 27-06-2012, 06:54   #3
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Originally Posted by denverd0n
Depends on what you mean by "safely." Deep cycle, lead-acid batteries will last longer if they are not routinely discharged too deeply. Generally speaking, the lower the average depth of discharge, the longer the batteries will last. So routinely discharging them more deeply may somewhat shorten their life.

On the other hand, on a dollars-per-watt basis, you may get more out of the batteries over their lifetime by discharging them somewhat more deeply than you have been. This is because it takes less time (and therefore usually costs less) to recharge from 50% to 85% or so, than it does to recharge from 85% to 100%. Figuring all those costs out, over the lifetime of the batteries, can be a fairly complicated calculation.

In any case, there is no immediate danger to discharging them to 12 volts. It's not like you are going to kill the batteries straight away by doing that. Lots of people routinely discharge their batteries to 12 volts (or even less) before recharging them, and still get many years of use out of them.
Thanks and a good simple answer 10 years or so mucking round with boats and I still can't seem to work out a complete service for battery's , I just fitted 3 new house 100 amp battery's with a current draw of about 7 amp in a 12 hour period they seem to get down to the 12.4 mark which I feel is nearly 50% seem rather quick but I guess it's about right.
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Old 27-06-2012, 09:32   #4
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Re: Batterys again

"the 12.4 mark which I feel is nearly 50% seem "
You need to make sure you are using an accurate voltmeter or battery management system. A fully charged battery is 12.6-12.7 volts, depending on the maker. Anything above that is just a surface charge. Every tenth of a volt below that, is about 10% of capacity. So a 12.4v battery is normally down about 20-30%, nothing near halfway. For the more common 12.6v battery, 50% down is 12.1V.

Assuming and ignoring many things like rate of discharge, temperature, and voltmeter accuracy.
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Old 27-06-2012, 14:12   #5
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Re: Batterys again

Open-Circuit Voltage State Of Charge Test:
For accurate voltage (OCV) readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs.
Disconnect all loads from the batteries, and measure the Open Circuit Voltage
90% SOC = 12.62 V OCV
80 % SOC = 12.50 V
70 % SOC = 12.37 V
60% SOC = 12.24 V
50% SOC = 12.10 V
40% SOC = 11.96 V

12 Volts is too low.
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Old 27-06-2012, 15:42   #6
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Re: Batterys again

Quote:
Originally Posted by southace View Post
Sorry I know this topic has been brought up many times but.......... My house 3 x 100amp battery's get down to 12.4 volt I start to worry. Can I bring them down to say 12 volt safely ? And then to charge them back full at 20 amp charge rate it will take 15 hours to charge ? Yes/no?
You may want to approach the problem differently. Your batteries have a certain number of charge cycles before they can no longer charge efficiently. The deeper you discharge, the fewer the number of cycles left in the bank.

At 12.4v you've used somewhere around 75 Ah. That's not such a big problem for a 300 Ah bank; depending on the type and quality of battery, you may be able to run them down like that every week and still have them hold out for six or seven years. However, if you run them down to 12v, you've expended almost twice as much power as running them down to 12.4v. (Check Gord May's chart if you don't believe me.) Start doing that on a regular basis, and you'll be lucky if those batteries last two full years.

That's the calculus that matters to most of us.
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Old 28-06-2012, 16:13   #7
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That depends on if you are a weekend warrior of full time cruiser/live aboard. Discharging to 12.0V on weekends and a couple of vacations a year won't hurt the lifespan of the batteries. Neither will doing it every day, the lifespan just gets used quicker....
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