Battery cable sizes - to 3 stage charger

northern_light · October 2011

Am new to forums and to solar power setups, so please be gentle....

I bought 4 PVX-490T SunXtender Deep Cycle AGM batteries (49 ah each) a couple of years ago, intending to set up my system. However, that didn't happen and the batteries have been sitting idle. I checked their charge and they are at 12.7 V. I realized I needed to charge them SOON, so have purchased a 20Amp Xantrex True Charge2 3 stage charger and use on-grid AC power to get them charged.

I have purchased cables to connect the 4 batteries in parallel - these are #4 cable/wire - arbitrarily decided on with discussion at the local auto marine supply store.

The specs that came with the charger say that, to connect the battery bank to the charger, #10 wire is sufficient (if run is less than 5 ft)

my questions;
Is it okay to have different sized wiring for this - #4 connecting the batteries to each other and #10 from batteries to charger?

Any help would be greatly appreciated.

Cariboocoot · October 2011

Re: Battery cable sizes - to 3 stage charger

Welcome to the forum.

There's no problem with using larger wire than necessary for any application, except cost. It's when you use wire that's too small that the sparks fly. Often literally. Just remember any circuit protection (and there should be either a fuse or breaker) should be in this case based on the maximum power of the charger: 20 Amps. That's a 25 Amp fuse/breaker max. That way if anything goes wrong the circuit protection will disconnect the power.

Big trick: getting four batteries in parallel to take current evenly. Please refer to the SmartGauge wiring charts here: http://www.smartgauge.co.uk/batt_con.html Method 2 would suit this application.

Hows the snow up there?

mike95490 · October 2011

Re: Battery cable sizes - to 3 stage charger

Since you are just doing a "top - off" charge, you can get away with it, but will have issues if you need to recharge after usage, in that configuration.

waynefromnscanada · October 2011

Re: Battery cable sizes - to 3 stage charger

I'm amazed that after sitting idle and unused for 2 years, they were not totally dead and ruined! Wow!

niel · October 2011

Re: Battery cable sizes - to 3 stage charger

you might want to go with thicker wire if you can as the voltage drop seen would be around .25v and that could fool the charger into thinking the battery is charged when it isn't. #8 for a 5ft run would be .16v dropped and #6 is .1v dropped.

now concorde does make some good batteries, but even i have trouble believing that after 2 years they are at 12.7v.

northern_light · October 2011

Re: Battery cable sizes - to 3 stage charger

Thanks for all the advice - much appreciated. I had already figured out that the configuration for the interconnection of the batteries (in parallel, with the cables to the charger from opposite, diagonal corners).
For this temporary setup - to get the batteries charged while I have access to AC grid power - is it totally necessary to put in the 25 amp fuse/breaker? (the specs say that it should be within 7" of the batteries.
And the next question will really show my newbyness - How?
The DC breakers I inherited with my cabin are large and made to use with the panel - and they're currently 100 kms away at my place. Are there other types to insert into the #10 (or maybe upgraded to #8 or #6) cable?
I sure wish one of the 2 solar guys I've tracked down in this area would have time - this DYI stuff might be beyond me!

thanks in advance
yes - snow on the ground - more coming today.

Cariboocoot · October 2011

Re: Battery cable sizes - to 3 stage charger

You know, for a temporary "see if the batteries are okay" application I'd just get some 10 AWG and a 25 Amp automotive fuse (perfectly good for a 12 Volt application) and hook up one battery at a time and watch its performance. Check the resting Voltage, run through a 2 stage charge (there's no need for the Float cycle in this instance) let it rest and check the Voltage again the next day. Your really just trying to find out if the batteries are still good. The step after that would be to purposefully discharge them and then recharge them again.

niel · October 2011

Re: Battery cable sizes - to 3 stage charger

just to clarify for no mistaking what i was saying, i was referring to replacing the #10 with thicker wire.

as to the fuse/breaker being necessary and close to the batteries, that is important as a short circuit along the wires could be disastrous with batteries being able to pump out great amounts of current. we don't know of the 7 inches, but it should be as close as you can possibly get it to the batteries to lessen the chances of shorts across the battery wires causing harm. it can be a cb panel mounted near the batteries and best secured on the outside of the battery box itself. i am assuming you have a battery box too as that helps to avert something happening to the batteries including the dropping of something metallic onto them that could short them out. even a wooden box would suffice as it does not have to be metal. some have even used heavy plastic containers too with some success.

i know it's hard to get a feel for things at first, but you'll get it as much of it is common sense and you will often say to yourself, "of course that makes sense. why didn't i see or think of that in the first place?";)

ps- first snowfall of the season here this day.
edit to add: funny we got snow and haven't officially hit the freezing mark as we have been as low as 1 degree above for 33 degrees f. there goes my garden.

northern_light · October 2011

Re: Battery cable sizes - to 3 stage charger

Okay - I'll try the "see if the batteries are okay" application to check each battery.

One more question (which I think I've read the answer to somewhere on here, but can't find it again)

I'm soon back at my off-grid place. Can I use my Honda Eu2000 generator to do this? Anything to be aware of? And if using the generator is okay, and I go to a full 2-stage charging, can I have another AC line coming from the generator at the same time to multi-utilize the electricity (to charge my little Eliminator battery pack, for instance, or generate good light during these shortening days/longer evenings)?

I know that I've read that some generators don't produce a pure sine wave, which might be harmful to batteries. Does the Honda 2000?

Thanks again

BB. · October 2011

Re: Battery cable sizes - to 3 stage charger

The 12 volt DC output on the genset is pretty much useless.

The 1,600 Watt (max continuous) is a very clean and frequency stable sine wave (much more so than regular generators). But won't support much more than its rated power (not much in the way of surge current).

The problem with AC battery chargers is a large number of them have a very poorly designed (i.e., cheap) front end that takes a lot more current than the Watt rating would imply... Basically, for a typical AC battery charger if you want want to run it from a eu2000i, the maximum output from the battery charger should not be more than ~800 watts, or:

800 watts / 14.5 volts = 55 amp @ 12 battery charger

Most people use around 40-45 amp maximum 12 volt battery chargers.

If you want to read about the details of AC battery chargers on a small genset, you can read this thread:

Question about battery charger selection with EU2000 generator

The 20 amp Xantrex TC is actually a very nice AC battery charger (power factor corrected) and will even run nicely on a eu1000i (and save your some fuel costs too). A single 20 amp charger is a bit "small" for the eu2000i and may tend to waste fuel (gensets drink fuel when powering small loads vs their rated output).

-Bill

Cariboocoot · October 2011

Re: Battery cable sizes - to 3 stage charger

The Honda has a really good, clean, stable sine wave. As good as any utility and better than some.

It will have no trouble running your 20 Amp charger and additional loads, so long as they don't total more than the gen's 1600 Watts maximum. The charger will draw about 250 Watts (it's a good charger, not one with a poor power factor).

northern_light · November 2011

Re: Battery cable sizes - to 3 stage charger

Cariboocoot wrote: »

You know, for a temporary "see if the batteries are okay" application I'd just get some 10 AWG and a 25 Amp automotive fuse (perfectly good for a 12 Volt application) and hook up one battery at a time and watch its performance. Check the resting Voltage, run through a 2 stage charge (there's no need for the Float cycle in this instance) let it rest and check the Voltage again the next day. Your really just trying to find out if the batteries are still good. The step after that would be to purposefully discharge them and then recharge them again.

Hello again - back in internet-land. I've charged 3 of my 4 batteries, using the 2-stage charge option and doing one battery at a time. The resting voltage was actually 12.3V (not the 12.7 I had said earlier). I used #10 wire with the 25 amp fuse. Each battery took about 5 hours to get to the 'ready' stage. I checked the voltage again the day - they're each at 12.74. Does this mean they're shot??
The Sunextender manual gives directions for a "Deep Discharge Recovery" for batteries in storage for a long period of time. But, it uses a constant current charge rather than a constant voltage charge.
You had also suggested purposefully discharging them, then see if they'll take a charge. Without having them hooked up to any kind of charge controller, how do I control the level they discharge to? Use an inverter (and load) with aun automatic low-voltage cut-off?

Thanks for any help.

Cariboocoot · November 2011

Re: Battery cable sizes - to 3 stage charger

12.74 is a good resting Voltage for a 12 Volt battery. It's a 'fully charged" Voltage. But that alone does not mean they're still good; hence the need to do a controlled discharge and recharge to see if they've still got capacity.

The tricky part is coming up with a way to discharge them over time; you don't want to do it too quickly. They're only 49 Amp hours each. What sort of inverter have you got? Sometimes the stand-by draw alone would be enough. For instance I'd test these with a 30 Watt (total) load and monitor the Voltage over not more than 10 hours. When it gets down near 12 Volts (with load) shut it off and see if it will recharge. This is by no means a perfect test! But if you haven't got a battery monitor or a load tester or any of the other fancy equipment it's about the best you can do.

Then after recharging you'll want to see if that resting Voltage stays up for a few days without significant change. If you find you can charge them to 12.74 but a week later they're back down to 12.3 there's a problem.

northern_light · November 2011

Re: Battery cable sizes - to 3 stage charger

I just bought a 600W Samlex inverter and have on hand a new 400 w eliminator. I assume that the the deliberate discharge test should be done on one battery at a time as well as the re-charging? (am doing this with a 2000w Honda generator, using the eco setting). Or can I connect 2 in parallel for either of these operations?

Thanks again

Cariboocoot · November 2011

Re: Battery cable sizes - to 3 stage charger

Do one battery at a time. I know; it's tedious! :roll: But if you try to test two at once, one of them could be good and the other one bad. Which is which? Then you have to start over.

The Samlex 600 has a very low idle current of 850 mA. So plug a couple of light bulbs into it that come up to around 30 Watts and watch the battery Voltage. If it falls to around 12 in about 10 hours that's good. If it falls to around 12 in about 10 minutes that's bad.

You get the idea; you're simulating a constant draw test like the one they use for giving batteries the Amp hour ratings. This isn't exactly that test, but it's the same principal.

If all the batteries discharge about the same over the same time then at least they're consistent. But don't be surprised if they're a little short on capacity after all this time.

Battery cable sizes - to 3 stage charger

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