Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

CALLD · August 2014

Hi there,

So before anyone jumps right in an state the obvious, I would just like to point out that I know I do not have the best set of batteries for the job but that it just so happened that I got stuck with these batteries from the start and that a new set of true deep cycle batteries will be added soon.

I've had my setup going now for 4 months and so far the performance has been pretty stable with no issues yet. The Panel array is a series/parallel setup to produce 36volts under load and up to 25Amps peak power. The series/parallel configuration was chosen due to shading problems in the morning and evening from nearby structures, and so keeps the power production at significant levels as long as sun is falling on at lease one string of 36v cells.

The charge controller is only a PWM as MPPT would not have made much of a difference due to the low voltage on the array.

The Batteries in question are 8x102AH 12V Maintenance free lead calcium batteries. They are marketed as "suitable for cycling duty" and are warranted to serve 250 cycles to 50%DOD. Problem for me is that nowhere on the battery does it say "Deep Cycle". It also specifies 710CCA and 180mins RC. This would lead me to believe they are actually SLI batteries.

The batteries are configured in 4 strings to give 24V and 408AH(C/20). The cables to the strings were painstakingly measured out so that each is exactly the same length to avoid any string carrying more current than the next.

The daily cycling depth varies between 20% to 40% and occasionally approaches 60% over 2 to 3 days in cloudy weather.

The maximum charge voltage was set at 2.5volts per cell, held for 2 hours before floating at 2.4 volts per cell.

Load voltages observed are as following:

+/- 100% SOC:
5A 25.6V
10A 25.4V
20A 25.1V
50A 24.7V
100A 24.4V

+/- 80% SOC:
5A 24.4V
10A 24.2V
20A 23.6V
50A 23.2V
100A 22.9V

+/- 50% SOC:
5A 23.8V
10A 23.5V
20A 23.1V
50A 22.9V
100A 22.4V

These voltages have pretty much been the norm since I started with the setup so there hasn't been any sign of degradation yet (the figures have been this low since the start).

Ok so what surprises me is that even under the biggest load which is only C/4 the voltages seem very low for the SOC. I have 2x7AH GEL batteries that I stripped from a UPS and they hold their voltages well above 24V even under a load of 5Amps (which is near 1C) for a significant amount of time.

I'm very curious to know what sort of working voltages people are experiencing with their off-grid systems - those with the proper deep cycle batteries. I'd like to see how mine compare, also maybe so that I can get a feel for how much longer these no-frills batteries can go for before they kick the bucket...

inetdog · August 2014

Re: Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

Your "maintenance free" batteries are, as you probably know, just FLA batteries with extra electrolyte and very hard to remove vent caps.
And the ones you have are indeed at best hybrid rather than deep cycle batteries. The spec of 250 cycles to 50% rather than 1000 or more is good confirmation of this. And that 50% discharge should be followed by recharging within a day or so to avoid permanent capacity loss.

To get really high discharge currents, you need a totally different construction with closely spaced plates that take up most of the interior volume of the battery. The AGM type can deliver high discharge currents and accept a higher charging rate (C/4 or higher depending on manufacturer recommendation) while GEL types can deliver high discharge current but must not be charged at faster than about C/20 to avoid permanent damage. That is OK for UPS use where the high discharge current is all that is needed. But batteries for PV need to be recharged over the course of 4-6 solar hours of daylight, and that just cannot be done with a GEL cell.

On the other hand, there is a lot to be said for inexpensive, non-deep-cycle batteries for your first set so you can learn before spending the big bucks on RE batteries.

Vic · August 2014

Re: Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

Your charge voltage seems a bit high. The Float voltage seems very high.

Did you get these voltages from the manufacturer of the battery or the Distributor?

Four strings of batteries, even with precisely identical cables, lugs and lug crimps will still cause troubles in time, as each cell of each battery may behave in a different manner than others in the bank - and, these differences will diverge as time marches on.

Are you using Temperature Compensation of the charge voltage?

As inetdog mentioned, 250 cycles to 50% SOC are not many for daily cyclic uses.

Guess that the +/-- headers indicate approximate SOC? And how was this nominal SOC determined? Cannot say much about your load voltage readings.

But, seems that you plan on switching to real Deep-Cycle batteries fairly soon anyway, so would just run what you have, until these batteries will not do what is needed of them. Kind of an obvious statement.

EDIT: Should add, that for Lead-Acid batteries, the ideal Absorption charge time (this is the stage that uses your 2.5 V/Cell value) depends upon the Depth Of Discharge that the battery experienced in the previous discharge cycle. It usually would vary, at least somewhat from day-to-day.

I have NO experience with this type battery in cyclic off-grid use.

Just opinions. Good Luck. Vic

CALLD · August 2014

Re: Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

Vic wrote: »

Your charge voltage seems a bit high. The Float voltage seems very high.

Did you get these voltages from the manufacturer of the battery or the Distributor?

Four strings of batteries, even with precisely identical cables, lugs and lug crimps will still cause troubles in time, as each cell of each battery may behave in a different manner than others in the bank - and, these differences will diverge as time marches on.

Are you using Temperature Compensation of the charge voltage?

As inetdog mentioned, 250 cycles to 50% SOC are not many for daily cyclic uses.

Guess that the +/-- headers indicate approximate SOC? And how was this nominal SOC determined? Cannot say much about your load voltage readings.

But, seems that you plan on switching to real Deep-Cycle batteries fairly soon anyway, so would just run what you have, until these batteries will not do what is needed of them. Kind of an obvious statement. Just opinions. Good Luck. Vic

Hi VIC

I couldn't get much info on charging algorithms from the manufacturer, probably because they don't want to divulge any more due to the fact that they are not true deep cycle batteries. However the standard practice for lead calcium batteries is a higher charge voltage. It is still winter here and because I only get a maximum of 7 hours of useful PV power on a clear day I have to make the most of it. I plan to reduce the voltage in summer when there are more sun hours. I keep the float voltage high because the charge controller doesn't allow me to set the absorption stage longer than 2 hours which I deem to be too short, so the float stage is essentially a reduced voltage extension of absorption. These lead calcium batteries have a very low absorption/float current. Once the charge voltage hits 2.5v per cell the current drops off rapidly to only C/50, at 2.4v per cell the current is as little as C/100. It leads me to believe they could actually do with a higher charge voltage. They batteries hardly show any rise in temperature at all during charge, a few degrees at most and that only if they were discharged more than 50% and had been rapidly charged by expeptionally bright sunshine.

Vic · August 2014

Re: Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

Hi CALLD,

Even for Lead-Calcium batteries felt that the Absorb V was a bit high -- this type battery if common for auto starting applications, and have run them in autos.

And believe that Gel and AGM batteries are Lead Calcium, and AGMs often run Vabs at about 2.4 VPC, FWIW.

Yes, there is little gassing on Lead Calcium batteries. Gassing is the major source of heating in conventional Flooded batteries -- much more so than heat from IR losses inside the batteries. However, Temp Compensation is also important in compensating charge voltages from the effects of the environment on the battery temperatures. Hope that you ARE using a BTS in this system.

Some SLA batteries do spes a very small finishing current, often such low values that current jitter from PWM or MPPT CCs might keep Ending Amps from being an effective method of ending Absorb.

Your Float voltage strategy may well be a good one, as some PbCa batteries spec 12-ish hour Absorption stages.

Good Luck. Vic

Does anyone have experience with Leisure/Marine/RV batteries in a PV System?

Comments

Categories