How long before sulfation?

[mtbandy]

For the last few weeks I've been running my solar system with a rather undersized SLA battery (17Ah deep cycle battery and 50W panel).

Anyway on good sunny days it is usually charged by lunchtime, floating at 14.2 volts on  the controller. Due to the size of the battery the voltage seems to drop rather a lot under heavy load, so the LVD on the controller kicks in before the battery is truely discharged. Because of this I tend to use as much power as possible when the sun is still out, and I'm not going to be draining the battery as much.

Anyway, back to the point... with careful usage, the LVD seems to kick in at about 10pm every night, and the system shuts down. At that point I switch everything off and leave it. Once the load is removed, the battery rises back up to about 12.2 volts, where it stays all night until the sun hits the panel the next morning. This battery is getting some serious abuse, about 80% cycling daily, sometimes even 2 full cycles per day yet it still seems to be working rather well! I'm just wondering how long a battery can sit in a discharged state before sulfation will set in in a way where the battery cannot be recovered by simply recharging it? Is the greater issue the depth of discharge, or is it the period that the battery is left in the discharged state?

[Nando]

It is extremely difficult to read your mind, so please give the proper information to be able to give you suggestions.

Sulfation will occur during discharge and during discharged times and the effect depends on the type, quality and of course ABUSE.

Most People MURDER their batteries, and You are doing it "VERY" WELL.

LVD trips but for Us is tripping in the dark since you are not indicating what the trip voltage is, to know the depth of the battery discharge level which can give you an estimate of the "sulfating" problem.

A 17 AH "deep cycle" battery under heavy load will "deep" down fast which forces the LVD to trip, also the internal resistance (impedance) of the battery is a bit high because the AH is low.

Solution: Increase the storage capacity of the battery.

You should have at least a 50 AH battery, you have 50 *6 hours = 300 watts divided by 12 Volts = 25 Ah * 2= 50 AH capacity.

75 AH may be a better solution.

Nando

[Ungrounded Lightning Rod]

Deep cycle batteries are only good for a couple hundred deep cycles.  Then they start to die.

That's why you try to have enough battery that they aren't all that far below full charge when the sun comes up again, and thus last for years, rather than sucking 'em down every night until the inverter cuts out, "freezing in the dark" until dawn, and having to replace the batteries before the year is out.

SLAs are especially problematic, since they'll outgas and die if you charge 'em too fast.  If you are really getting 50W from that panel you'd be charging them at 1/4C (a quarter their capacity per hour).  You're probably actually charging them at about 1/5C.  They should be charged at no more than 1/10C.  So you are charging them at twice what should be the maximum rate.

In your position I'd get at least two 100ish amphour deep cycles for at least a couple hundred amphours capacity - then turn the lights off at 10 PM anyhow, until I got another panel.

[mtbandy]

sorry the LVD cut off is 11.8 volts, not sure about the hysterisis on that because I haven't noticed when it has come back on. With a 7A load, I'm sure the LVD trips 'too early'.

I know I'm mistreating this battery, but I just wondered by what extent. A few years ago, I used to go on regular mountain bike rides at night. A couple of us used small SLA batteries to power our lights, and these were completely dead by the end of the night (down to about 10.5 volts). We knew nothing about batteries back then, and just stuck them back on charge the next day. They lasted ages like this, in fact I never saw one fail! That's the only reason I was asking, just wondered.

I'll be buying myself a battery bank tomorrow hopefully, around the 200Ah mark so I can give it very shallow cycling and stop killing this small thing!

[Flux]

Batteries die for many reasons, you can't win really.

If you never use them but keep them fully charged they die from grid and interconnection corrosion.

If you don't use them and don't keep them charged then they sulphate and die.

If you cycle them over a large range then you wear the plates out quickly. The best compromise seems to cycle them about 50% discharge and make sure that they are completely charged at least once each week. Leaving them without a complete recharge at least weekly will probably cause sulphating at some stage.

Less than 50% discharge will give longer life, but not necessarily better value for money. They have a limited life anyway so it pays to use them to a reasonable extent before they die of old age.

If you have the luxury of wind or sun each day you can manage with a smaller bank. In many areas the source of charge is not that predictable and you need an excess capacity to cover several days of negligible input. Too big a bank means that you never charge it often enough and it sulphates. For critical systems with a well sized bank some form of back up charging is still required in most locations for the odd calm and cloudy weather.

With your little battery and that cut off voltage you are probably not cycling them as deep as you think, but more capacity will give you a longer life ( and probably better value for your money).

Flux

[wooferhound]

Here is a long thread about battery life . . .

http://www.fieldlines.com/story/2005/4/2/63834/21217

[mtbandy]

That was interesting!

Cycling this small battery won't be a problem anymore, as I am now the proud owner of a 344Ah bank! I know this is oversized for my 50W panel, so I plan to eventually use an intelligent watt-hour meter (new project!) to ensure that what's coming out can get back in the next day. I won't be cycling it very deeply at all. I'll save all that for the diary anyway...

Thanks for the replies

[BigBreaker]

I am coming around to Ni Fe batteries as the solution to the cycling problem.  Importing them from China would be annoying but a battery with a 50yr life that can take abuse is awesome.  The cost per usable kwhr is similiar to lead acid, but the efficiency is lower.