SLA battery, load test. fail or not?

[Norm.Mareeba]

Hi all;
This is probably off base (wrong forum) but I've got an electronics puzzle that I'm not making much progress at...

I've got an electric/rechargeable ride-on mower that has been working quite nicely for the past five years. But I'm the last six months I haven't been able to do the whole lawn without stopping for a recharge. I suspected that the batteries are just getting tired...

It's got three JIUHUA 6-CNF-38 (12v-38ah) deep cycle batteries, wired in series, making 36 volts. At $160 per battery, I'm not rushing out to buy new batteries without proving to myself that they are indeed, tired.

I took the batteries out and charged them up individually with my 12V battery charger. The all charge up normally, settling down to a normal, less than 1 amp trickle charge. They held the normal charge of 13+ volts overnight.

OK, so I reasoned that what I needed was to see how they acted under load. So I set up a 12V-60W headlight to act as the load and my volt-meter to monitor the voltage over time. I recorded the voltage at 5 minute intervals. The number are as follows (T is elapsed time).

Battery 1 .... T0=13.22v, .... T5=10.68v, .... T10=10.64v, .... T15=10.55v, .... T30=10.31v
Battery 2 .... T0=13.57v, .... T5=12.53v, .... T10=12.55v, .... T15=12.54v, .... T30=12.5v
Battery 3 .... T0=13.31v, .... T5=12.05v, .... T10=10.64v, .... T15=10.52v, .... T30=10.5v

OK, so what does this show? I've got one battery that settles down to 12.5 volts under a load and two batteries that quickly drop to 10.5 volts and stay there...

Which is right? ..are they both OK and my problem is elsewhere?

Thanks, Norm.

[Bohdan]

It looks like you have one weak cell in two of the batteries. The problem with pushing batteries that are made up of multiple cells until they are flat is the there is always a weaker cell that goes flat early. Once that happens and you keep using the battery the weak cell is reverse charged by the current flow and this tends to destroy the weak cells. They never recover. If you have a battery made up of 2V cells stop using it as soon as the total voltage drops by 2V and recharge it.

That is the reason that the better, more expensive, batteries last longer because they are made up of matched cells and there is no significantly weaker one.

[Norm.Mareeba]

It looks like you have one weak cell in two of the batteries. That is the reason that the better, more expensive, batteries last longer because they are made up of matched cells and there is no significantly weaker one.

Thanks, makes sense to me.

So, the one that is maintaining 12.5V over a period of time is the way they are supposed to act.. Yeh, I didn't have a choice in batteries - they came with machine, but I did suspect them since they were Chinese. Same with the machine, had to tighten bolts, repair wiring, pack bearings, you name it, I fixed it. I'm better at keeping electrics going, vs a gas mower. You takes your choice, you pays the price!

Thanks..

[malb]

100% with Bohdan, two batteries each have one failing/failed cell in them.

Some things to remember with all SLA type batteries;

The amp hour rating is normally specified for a constant current load discharging over 20 hours. This gives the highest 'claimable' capacity. For yours, that implies that that the spec for them was that they should maintain a specified voltage (often 11.5V) after being discharged at a constant 1.9A for 20 hours when new. Some manufacturers may also include capacity specs for higher discharge rates, but if they do the capacities fall fairly rapidly as the time to discharge decreases and the discharge rate increases.

This class of batteries tend to have a lifetime expressed in charge/discharge cycles rather than time. The standard for this is for 20% discharge each cycle, so the battery starts fully charged, and is discharged by 20% of its rated capacity for each cycle, then recharged again. Some high end manufacturers will supply additional data for heavier discharges, but the total lifetime falls as the depth of discharge increases. A high quality battery may handle up to 3000 cycles at 20% depth of discharge, while an el cheapo might struggle to handle 1000. Extend the depth of discharge to 50% and the numbers might be closer to 1000 and 150.

[Norm.Mareeba]

........ while an el cheapo might struggle to handle 1000. Extend the depth of discharge to 50% and the numbers might be closer to 1000 and 150.

That's shocking! Not your numbers, mine! I've had the mower for five years, mow the lawn every week. That's 52 weeks times 5 years = 260 charge cycles. Nowhere near even your worst case scenario for an el cheapo.

OK, maybe this isn't the place but here is an AGM battery (not a SLA). I could make it within my budget. Neuton Power NPD12400 12V 40Ah AGM Deep Cycle Battery Golf Buggy Scooter | eBay (http://www.ebay.com.au/itm/Neuton-Power-NPD12400-12V-40Ah-AGM-Deep-Cycle-Battery-Golf-Buggy-Scooter/222553759929?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649)

Any opinions?

Norm

[Norm.Mareeba]

OK, maybe this isn't the place but here is an AGM battery (not a SLA). I could make it within my budget. Neuton Power NPD12400 12V 40Ah AGM Deep Cycle Battery Golf Buggy Scooter | eBay (http://www.ebay.com.au/itm/Neuton-Power-NPD12400-12V-40Ah-AGM-Deep-Cycle-Battery-Golf-Buggy-Scooter/222553759929?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649)
Norm
Went up to out local battery palace here in Mareeba (Brunos) and got three AGM 12V45AH. $140 each, not $130 as on eBay. I don't mind paying the $10 extra especially when I'm supporting a local business. Made in China but, if something goes wrong, I've got someone I can work with.

Thanks for your help,
Norm

[malb]

That's shocking! Not your numbers, mine! I've had the mower for five years, mow the lawn every week. That's 52 weeks times 5 years = 260 charge cycles. Nowhere near even your worst case scenario for an el cheapo.

Norm

Given that you are going to be drawing way more current than the 1.9A suggested for a 20 hr discharge rate (1st para), I think 250 cycles is quite good for the original set. Glad that you bit the bullet and replaced the full set, rather than just the 2 with the failed cells as the better one of the original set would also have had some issues with overcharging and being overworked in the original set and would not match well to the newer ones, ultimately causing them to work harder to make up for it's deficiencies, and hence shortening their life as well. At least with 3 new batteries, they should have similar characteristics and hopefully set you up for a trouble free run for quite a while.

[Chas]

Went up to out local battery palace here in Mareeba (Brunos) and got three AGM 12V45AH. $140 each, not $130 as on eBay. I don't mind paying the $10 extra especially when I'm supporting a local business. Made in China but, if something goes wrong, I've got someone I can work with.

Thanks for your help,
Norm

Hello Norm,

I hope your battery charger is fully compatible with your new AGM batteries, which may have a different charging profile from the (presumably) GEL SLA units they are replacing. AGM batteries don't like to be overcharged. It would be wise to check with the battery specs to make sure your charger is suitable.

Chas.