# Batteries never reach bulk charge

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Registered Users Posts: 9
I have a off grid solar system. Using a Xantrex C40 controller hooked up to 8-6v Trojan 105 batteries. They are hooked up in 4 banks of 2 batteries for a 12v system.

My problem is my batteries never seem to charge fully. I have the Bulk voltage set to 14.8v. The voltage on the controller never get above about 13.4. The status light on the control never blinks more than the slow "one blink" status.

I have tested the voltage coming directly from the solar panels and it is at 20v.

## Comments

• Solar Expert Posts: 69 ✭✭
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Batteries never reach bulk charge

Let's say the Trojan T-105 is rated at 220 AmpHr, you have 4 parallel strings,
for a 12 volt bank, @ 880 Amphours.
Your C40 does NOT have sufficient charging amps for that size battery bank.
The reasonable minimum charging amperage of 10% of the C/20 rate is 88 Amps.
If your batteries are not totally sulfated up from continuous deficit cycling, you MIGHT be able to charge them in single series pairs with your equipment.
IF the batteries are salvageable, and IF they attain full charge, you will be looking for at least 80 and preferably 100 AMP charger to maintain them.
If you need that much battery, you may well benefit from a higher voltage configuration, either 24 or 48 volts.
Many here will recomend 48 volt systems, and you already have 8 batteries in hand.....
For my own system, I operate at 24 volts, a compromise that suits my needs very well.
• Banned Posts: 17,615 ✭✭✭
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Re: Batteries never reach bulk charge

As Fullpower said, even if the charge controller were "maxed out" it wouldn't be able to supply enough current to properly charge that 880 Amp hour 12 Volt bank.

Moreover, you would need 623+ Watts of panel to reach that 40 Amps @ 12 Volts
To actually charge that bank properly you'd need 1371 Watts of panel and two charge controllers.

So let's go at it from the other direction: do you have numbers for how much power you'll use? That way you can determine how much battery you really need and what would be required to recharge it.

You probably do not need 5 kwh of power potential. If you do, higher system Voltage is definitely in order.
• Solar Expert Posts: 3,009 ✭✭✭✭
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Re: Batteries never reach bulk charge

Was going to ask what you had for PV, but in a roundabout way, cariboocoot just did, bit I have one more question - - -
That 20 volts you measure at the panels was that measurement taken while the panels were under load, while the battery voltage was at the stated 13.4 volts, or even a lower battery voltage? If so, how long are the wires between the panels and the controller, and between the controller and the batteries? And what size are those wires? You definitely should not be loosing almost 6 volts between the panels and batteries.
And what wattages are the lights etc and how long each day are they being used? Over sized battery bank aside, you may well be using more power than your panels are capable of replacing.
• Registered Users Posts: 9
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Re: Batteries never reach bulk charge

This system is being used in a cabin like setting. It has 4 days to charge and is being used for ~3days. It is running florescent lights 3 ceiling fans a TV and stereo. The 20V was not measured under load. The total amperage draw if everything is running is 6.2amps. It is rare that everything would be running at once.
• Banned Posts: 17,615 ✭✭✭
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Re: Batteries never reach bulk charge
deltarat wrote: »
This system is being used in a cabin like setting. It has 4 days to charge and is being used for ~3days. It is running florescent lights 3 ceiling fans a TV and stereo.

That "four days to recharge" method does not work well. Especially not with a very small amount of panel.

What you run is one factor. How long it runs for is another. For instance one CFL light may be 13 Watts. Leave it on for six hours and you get 172 Watt hours. That's about the same as running a TV for one hour.

My off-grid cabin runs on about 2.4 kwh per day. That's running a full-size refrigerator, computer with satellite modem, lights, water pump, et cetera. Quite a lot of equipment. On a 12 Volt system that would be 200 Amp hours, which would be a minimum of 400 Amp hour battery (no greater than 50% DOD).

If you have some Watt numbers from the equipment and some hours of usage we could get a rough estimate of your daily kwh use.
• Registered Users Posts: 9
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Re: Batteries never reach bulk charge

I am using 2 205w 12v panels. It is being converted to 120V through an inverter. IF I ran everything for 8 hours I would be using 6.2KW. (please let me know it my math is wrong) Obviously I am a rookie! Are you saying it would better serve me to have more panels and less batteries? And a different charge controller?
Again, please forgive me for my lack of knowledge.
• Banned Posts: 17,615 ✭✭✭
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Re: Batteries never reach bulk charge

Well let's see.
410 Watts of panel is about enough to recharge on set of those batteries (220 Amp hours @ 12 Volts).
That would supply at most 1320 Watt hours.
If your usage is place near even 1/3 of your 6.2 kwh number you're deficit charging.
If your usage is 6.2 kwh the battery bank you have (880 Amp hours @ 12 Volts) wouldn't last one day.

The first step is, as always, to get a handle on the power usage. It's time for a Kill-A-Watt meter and some accurate measurements of how much each thing consumes during a day of typical use.

The second step is to disconnect those batteries. You should try using only one set at a time. Charge them all up somehow, even if you have to buy a generator and cheap automotive charger to do it. You've got a problem here where the whole bank together may be sitting below 75% SOC for days on end, and that's not good.

I think it's inevitable that you will need more panels, unless you can severely curtail the power usage.
• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: Batteries never reach bulk charge

FYI,

We live off grid, we use ~5-700wh day. We charge with 400 watts of panels, into 4 t105s wired for 12 vdc. We charge at ~ 20 amps on average. Daily draw is about 10-20% of ah capacity. On balance, the system is quite well balanced between loads, battery bank size and charging capacity (PV)

Tony

Ps Remeember, all design considerations begin with the loads! Doing it anything else is folly.

T
• Registered Users Posts: 9
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Re: Batteries never reach bulk charge

Well, you have given me alot to think about. I will get a Kill-a-watt meter and play with my battery configuration this weekend. Thanks for the help.
• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: Batteries never reach bulk charge

Another quick rule of thumb for off grid, which aids in design because it is simple.

To define how much pow you can harvest from a battery bsed PV system do this.

Take the name plate rating of the PV divide that by 2 to account for all cumulative syst loses, then multiply that number by 4 to account for the average number of hours how good sun one could reasonably expect to harvest, per day on average over the course of the year. Some days better, some worse, but 4 hours has proved to be. Prty good average in most latitudes and climates. ( see also my numbers!)

So in your case, 410/2=205*4=820 watt/hours/ day. So with your PV shoot for a load of slightly under 1 kwh/day. Adjust the PV for the load, or the load tot he PV, and then adjust the battery to that. Add water and bake for a couple of years, and you will have a Greg PV system.

Tony

PS. Y ou should Walsh realize that most "newbies" wreck at least one set of batteries before they get it right.

I suggest y. Read the following links:http://www.batteryfaq.org/
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

T