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I have a lead-acid battery : HANKOOK DC24MF DEEP CYCLE 80Ah 12V.

In hope of extended this battery's life, I was thinking about buying an identical one and put it in parallel. This way by using "80Ah", instead of emptying one battery, both will drop to 50%. (Simplifying the math here of course.)

1 - Is this method good to extend both batteries life ?

2 - They are rated for 550 cycles, but emptying only half doesn't count as a full cycle right ?

3 - I considered 12V as "Empty" and 14.2V as "Full". Are these good numbers to save battery life ?

EDIT: Battery would loose warranty after 4 years or if below 10.6V

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    \$\begingroup\$ They are rated for 550 cycles Sure but from my experience: an SLA battery in a UPS is (trickle) charged continuously. After 5 years the battery has worn out (less than 20% of original capacity left) even though it has gone though less than 5 full charge-discharge cycles. My point: how the battery is used can have a significant impact on its lifetime. For my UPS example, using two batteries in parallel doesn't help much. It is better to just keep the money and buy a new battery when the 1st one has worn out. \$\endgroup\$ – Bimpelrekkie Jul 29 '19 at 12:59
  • \$\begingroup\$ I will use it almost non-stop as a buffer (ideally using as much Watts as generated) between my solar panels and my electric system. (On load ~14h a day.) Are you saying that keeping it almost at full charge will damage it as quickly as full charge / discharge cycles ? \$\endgroup\$ – bob dylan Jul 29 '19 at 13:03
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    \$\begingroup\$ SLA batteries are anything but linear and are really quite complex. But if they were linear, then 2*550 = 1100 days or about 3 years. In reality probably more like 1 year. I wonder if Maxwell energy-storage capacitors might be a better fit for this. Higher initial cost but longer life expectancy. \$\endgroup\$ – rdtsc Jul 29 '19 at 13:17
  • \$\begingroup\$ That's extremely expensive, I would rather try to save bucks here and play it smart. I might go Li-on in the future, if the prices are reasonable. (but now 1000$ for a 80Ah no thanks.) \$\endgroup\$ – bob dylan Jul 29 '19 at 13:39
  • \$\begingroup\$ There are many more battery technologies on the horizon. HP and others have been working on solid glass cells, promising 10x the performance of current. We'll see... \$\endgroup\$ – rdtsc Jul 29 '19 at 14:53
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1) Fully discharging a lead acid battery is bad for its life, going to only 50% is good, and will extend both batteries lives. Twice the installed capacity also gives you an emergency 100% reserve, if you're happy for that capital to be tied up permanently.

Before you connect two batteries in parallel, they must (obviously?) be the exactly the same chemistry and voltage, no paralleling a wet 12v and an SLA 12v. They should also have the same voltage, equalise them with a resistor or a lamp if not. It helps if they are the same age of battery.

2) Cycles are usually counted as a period of discharging, followed by a period of charging. It doesn't matter how deep this cycle it, it still counts as a 'cycle'. However, the battery life will be likely extended to thousands of these shallower cycles. It's more than word games, as battery life is non-linear with depth of discharge. If you start counting fractional cycles, then a 50% DoD might have to count as 0.2 of a cycle.

3) You might want to review that 14.2v figure for how you intend to charge the battery. It's a bit high for float charging SLAs, and a bit low for cyclic charging wet cells. Too low a charge voltage can lead to sulphation, which is another way to shorten the life of the battery. 12v as an endpoint will give you better lifetime than the 11.5v or even 11v that I've often seen quoted in battery literature.

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  • \$\begingroup\$ I edited my post. Are you saying that question 1=yes, 2=yes and 3=yes ? \$\endgroup\$ – bob dylan Jul 29 '19 at 13:25
  • \$\begingroup\$ @bobdylan I've edited my answer \$\endgroup\$ – Neil_UK Jul 29 '19 at 13:38
  • \$\begingroup\$ 3) My PWM charge controller actually had a 14.8V threshold for stopping the charge of the battery. (I guess maybe they take into account voltage changes for both charging and load.) I manually changed it to 14.2V because some people say 14.8V can be harmful to the battery's life. (electronics.stackexchange.com/questions/446201/…) - If no charge and no load (during rest), the voltage of the battery drops to 13.8V maximum, even if charged with 14.8V. - According to cheap PWM. \$\endgroup\$ – bob dylan Jul 29 '19 at 13:46
  • \$\begingroup\$ For posterity reading this post : When adding a new battery I think you should ideally have a similar model and charge both of them individually to the same voltage before connected them in parallel for twice the Ah. (Correct me if I'm wrong :P.) \$\endgroup\$ – bob dylan Jul 29 '19 at 13:53

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