SLA Batteries ---safe discharge voltage??

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PJIII
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Hi, I have a new set of four 33 amphr SLA batteries configured series/ parallel for 2-24 volt cells. I am using them in an electric trike and want to preserve their life as long as I can.

What is a safe and reasonable discharged voltage?--(the voltage at which I should recharge them) I keep hearing "don't discharge over 30%"---well, 30% of WHAT??? 30% of 12 volts is 4 volts & 12-4 =8 & I KNOW they don't mean 8 volts. I just need the voltage below which I should not let them go. Thanks for your help!

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nogaselectric
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Re: SLA Batteries ---safe discharge voltage??

Every battery mnfr. has their own specs so best to go to the source for an answer to your question. If that is not an option you can go by these rules of thumb. 10.5 volts is the lower limit you want to discharge a 12v battery safely. There is some normal voltage "droop" when the batteries are placed under a load i.e. take offs or hills. Typically if the voltage drops under 10.5 for lets say about 5-8 seconds under a take off on level ground its time to recharge. Try hooking up a volt meter somewhere thats easy to read so you can accurately monitor the voltage as you ride. The nice thing about an SLA battery is that typically its voltage drop curve is nice and smooth so its pretty easy to see what state of discharge your battery pack is in via a volt meter. Notice how far it drops on take off and where it ends up once you reach full speed on level ground. By monitoring these reading throughout a discharge cycle you will become familiar with your pack.Hope that helps.

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Spaceangel
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Re: SLA Batteries ---safe discharge voltage??

It is not so much voltage but also Amp Hours. Use an Amp meter to measure current and if you have a series parallel set then the amount of proper draw would be like 33x2=66 AH and 66 amp for one hour or 33 amps for 2 hours or 3.3 amperes for 20 hours!!! Actually it is closer to 3.3 for 20 and not 66 for one hour.

Hi, I have a new set of four 33 amphr SLA batteries configured series/ parallel for 2-24 volt cells. I am using them in an electric trike and want to preserve their life as long as I can.

In general one can go below 10 or 10.5 volts but not for too long. Battery capacity is measures in AH 20 Hour BCI and if you Google rates of Pb-acid batteries you can get the 10 hour rate and 5 Hour rates, even the 3 hour and one hour rates but the amount electricity you get is like 1/2 to 1/4th the original capacity. I am doing a good job of killing my Greensavers for I am going to replace them anyways. Some people get a year or two out of them. One can get only so much power out of a battery so draw current slowly if possible.
What is current draw? What size controller are you using? What is average HIGH current draw?
Rusty

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nogaselectric
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Re: SLA Batteries ---safe discharge voltage??

The last post is accurate and takes it to the next level. If you do have an amp meter at you disosal and want to accurately monitor your pack to that degree check out a cycle analyst. Its a handy little device that once calibrated will tell you how many amp hours you have used up during a discharge. Its fairly easy to install and works well with surprising accuracy when calibrated correctly. All of this stuff takes a little trial and error at first, but once you get a handle on it its smooth sailing/riding.

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sangesf
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Re: SLA Batteries ---safe discharge voltage??

First off get a good watt meter, put it inline between the battery and controller (use 12awg wire) and when at rest if the voltage of the battery pack is below 23.70 (2 x 11.85) then stop power and recharge asap.

You don't want the voltage to drop below that because that's 80% used up and if you recharge using no higher than a 10a charger then the batteries will last about 800 times.
Drop the voltage below that and/or recharge it at any higher amps then you will lose approx 1 recharge cycle everytime.

Let's say if you drop below the 11.85 per (23.7v) or charge over 10a maybe 40 times over a 2 year period that will be about the lifespan of the battery.

Of note: the 33ah batteries should give you a mile per every 1.5 ah...
So for your 2 battery packs for a total of 66ah, you should get about (66ah / 1.5ah) of 35 miles.

PJIII
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Joined: 03/26/2010
Points: 82
Re: SLA Batteries ---safe discharge voltage??

Thank you ALL for your comments---all very good information that I was needing. I am now considering installing either a Watt's-up meter or a Cycle analyst. Also re: range---you are correct--I am getting approx 45 miles at 23.91 volts discharge level w/ no hills. The latest test included four bridge (hill)crossings for 25 miles at 24.58 volts discharge level.

I have also noticed a marked increase in performance of the batteries as I break them in & that was predicted by another source. For example early on I traveled 25 miles on flat roads discharge level at 24.48 and yesterday traveled 25 miles w/ 4 bridge crossings (hills) with discharge level at 24.58 Volts.

I have 12 forward speeds via gears and have noticed that the motor runs much more efficiently at full throttle than anything less so now I tend to control the speed via the gearing & run with the throttle wide open. I also use the lowest gear when climbing hills.

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antiscab
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Joined: 07/07/2007
Points: 1686
Re: SLA Batteries ---safe discharge voltage??

nogaselectric wrote:

If that is not an option you can go by these rules of thumb. 10.5 volts is the lower limit you want to discharge a 12v battery safely

the 10.5v rule only works for a single 12v battery.
if the setup is 24v, the limit becomes 22.5v.

heres the reasoning:
at the cell level, *everytime* you go below 0v the cell gets damaged (well worn much faster than usual), regardless of state of charge (its because its max current has been exceeded, the max current depends on state of charge and state of health).

measuring voltage at the 6-cell level, 10.5v gives a worst case scenario of 5 x 2v and the 6th cell at 0.5v.
at the 12 cell (24v) level, 21.5v gives a worst case of 11 x 2v and the last cell at 0.5v

sometimes you can take the pack voltage lower without damage, sometimes not.
you just have to make sure no cell goes past that 0v. that includes at peak discharge.

to go to lower voltage reliably either takes monitoring at lower cell count levels, or just being lucky (balanced pack all of same Ah).

Matt

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