Charging an auxiliary battery from solar bank

[SDC]

Hi guys,

I have a 12V battery bank consisting of 4x US2200 6V 232Ah batteries from which I would like to charge (in fact, mostly maintain the charge of) a small starting battery for my ATV. The reason for this is that while sitting for a long time in cold weather, my ATV battery gets down low to a point where it won't crank the engine anymore. I was therefore thinking about a way to somehow take some energy from my solar bank to keep my ATV battery charged at all times. Here are the options I thought about.

1: Purchasing one of those small trickle charger (2A or so). Problem with that solution is that those chargers are 120V so I would have to let my inverter on while I'm not there...Really bugging as far as I am concerned.

2: Using the auxiliary 200 mA output from my outback flexmax 60 CC with a fuse to protect the controller. Also bugging because that is not enough current and I really don't want to mess around with my flexmax...plus, it is not reliable (I think the fuse would blow pretty easily and wouldn't charge my ATV battery anyways...)

3: Finding a 12V trickle charger that would have a 12V input. Anybody ever heard of such a charger.

4: I was thinking about one of those battery isolators commonly used in marine applications. I really don't know how they work and what are the implications of using this kind of device but to me, it sounded like the best option. My idea what then to wire this isolator via a properly sized breaker from my DC subpanel... What do you guys think of this?

Thanks for your ideas!

[inetdog]

Hi guys,

I have a 12V battery bank consisting of 4x US2200 6V 232Ah batteries from which I would like to charge (in fact, mostly maintain the charge of) a small starting battery for my ATV. The reason for this is that while sitting for a long time in cold weather, my ATV battery gets down low to a point where it won't crank the engine anymore. I was therefore thinking about a way to somehow take some energy from my solar bank to keep my ATV battery charged at all times. Here are the options I thought about.

1: Purchasing one of those small trickle charger (2A or so). Problem with that solution is that those chargers are 120V so I would have to let my inverter on while I'm not there...Really bugging as far as I am concerned.

2: Using the auxiliary 200 mA output from my outback flexmax 60 CC with a fuse to protect the controller. Also bugging because that is not enough current and I really don't want to mess around with my flexmax...plus, it is not reliable (I think the fuse would blow pretty easily and wouldn't charge my ATV battery anyways...)

3: Finding a 12V trickle charger that would have a 12V input. Anybody ever heard of such a charger.

4: I was thinking about one of those battery isolators commonly used in marine applications. I really don't know how they work and what are the implications of using this kind of device but to me, it sounded like the best option. My idea what then to wire this isolator via a properly sized breaker from my DC subpanel... What do you guys think of this?

Thanks for your ideas!

For a trickle charge (battery maintainer) by far the simplest and most reliable solution will be to get a suitable size battery maintainer panel (output up to ~18 volts without load and current output so low that you do not need to put a CC between the panel and the battery. A 20 watt panel may be good enough for you. We need to know the capacity of the battery in Amp-Hours (AH).
If it is a 70AH battery (not unusual for an automotive cranking battery) you should shoot for a current between .35 and .7 amps. That would be an 8 to 16 watt panel after you allow for the lower current that wattage corresponds to at a higher voltage which will do you no good. 20 would be OK too.

Anything you try to do involving your existing panels and battery bank will be needlessly expensive and complicated, as well as a pain to keep up and less reliable. The battery maintainer panel can even travel easily with the vehicle from one parking location to another as long as you get good sun.

[SDC]

For a trickle charge (battery maintainer) by far the simplest and most reliable solution will be to get a suitable size battery maintainer panel (output up to ~18 volts without load and current output so low that you do not need to put a CC between the panel and the battery. A 20 watt panel may be good enough for you. We need to know the capacity of the battery in Amp-Hours (AH).
If it is a 70AH battery (not unusual for an automotive cranking battery) you should shoot for a current between .35 and .7 amps. That would be an 8 to 16 watt panel after you allow for the lower current that wattage corresponds to at a higher voltage which will do you no good. 20 would be OK too.

Anything you try to do involving your existing panels and battery bank will be needlessly expensive and complicated, as well as a pain to keep up and less reliable. The battery maintainer panel can even travel easily with the vehicle from one parking location to another as long as you get good sun.

Thanks Inetdog for your reply. This is actually a pretty good solution. The reason why I don't really want to go with it is that I would have to install that battery maintainer panel quite far from my ATV and thus I think I would loose quite a bit of power in the wires...Also, I would have to install it on top of my three 200W panels and I think it would look kind of weird to have that small panel sitting there...

I was looking at those isolators and I can get one for 60$. So I find it pretty decent and it would prevent me from having to go on the roof again to install the maintainer panel (that would also be around 60$...).

My only concern is to make sure that such a battery isolator has no chance to do any arm to my bank...As far as it is simply a diode letting the current flow only in one direction, I wouldn't think so but I really want to make sure before I go ahead with this...

Thanks again!

[Bala]

There are DC to DC chargers available for Automotive.

You could use the Aux output from your charge controller and a relay and set on/off voltage limits ( On mine I can) for the Dc to DC charger so it will only be on when your batteries are at a charged level.

The ATV battery would not need to be on charge all the time.

[Mike90250]

What sort of solar do you currently have on your main 12V bank ?

Why not look into a small 10A ballpark charge controller, wire in parallel to your PV array, and use it to keep your small battery topped off , instead of a $60 isolator, get a $58 charge controller. You may
need to tweak it's absorb voltage setting to provide "Float" charge instead of "Cyclic service" charge.

[inetdog]

Thanks Inetdog for your reply. This is actually a pretty good solution. The reason why I don't really want to go with it is that I would have to install that battery maintainer panel quite far from my ATV and thus I think I would loose quite a bit of power in the wires...Also, I would have to install it on top of my three 200W panels and I think it would look kind of weird to have that small panel sitting there...

I was looking at those isolators and I can get one for 60$. So I find it pretty decent and it would prevent me from having to go on the roof again to install the maintainer panel (that would also be around 60$...).

My only concern is to make sure that such a battery isolator has no chance to do any arm to my bank...As far as it is simply a diode letting the current flow only in one direction, I wouldn't think so but I really want to make sure before I go ahead with this...

Thanks again!

You are welcome.
Point number one: If the current is only .5A, then the power lost in the long wire can be made very small.
Point number two: Since you would have 18 volts or more available from the panel and you would only need 14.5 volts or so to do the job, any voltage drop less than 3.5 volts would not cost you anything at all. No performance loss. That would allow you to have 7 ohms resistance in the wiring, and that is an absurdly large amount even if you use 18GA wire.

A diode would allow current to flow only into the ATV battery, but would not do anything to regulate the charging current to avoid damaging the battery. You would still need a CC with current limiting. And you would not want that CC to stay connected when you drag the starting battery down during starting. A diode isolator would not help for that either.
All it would do would be prevent the main battery bank from sucking power from the ATV battery if it went to too low a voltage during heavy loading or prolonged rain and shade. .

[SDC]

Well, thanks for all the inputs!

You guys are right, the battery isolator idea is useless...

I really like the idea of using the auxiliary output from my CC (with the configuration of a low voltage disconnect) with a relay to power either:

1- A DC to DC battery trickle charger as Bala suggested (do you know where I can find one of those and how expansive they are?)

2- A small inverter (on which I would leave the switch on) and a regular 120V multistage 2A trickle charger (I know this solution is kind of stupid in terms of efficiency but since I already own these two items, it would only cost me a relay, fuse and some wire...And with a low voltage disconnect on the CC output, no chance to drain my bank...).

Mike 90250: I have 600W of solar supplying the CC and the bank, are you suggesting that I could simply hook up another PWM CC in parallel to my Outback and use this small CC to charge my battery. It sounds to me like a pretty good idea although I am a bit concerned not to damage my Outback CC...Do you think it could cause any damage on my CC?

Inetdog: You are right, it is kind of ridiculous to think about losses in wires in that range of power. You are also right about the isolator, it is useless. However, it would still be a bit of a pain for me to install a small panel on top of my other panels and probably look weird too. So, as I mentioned above, I really like the idea of to DC to DC charger via a CC aux. output powered relay. Just leaves me with the task of finding one!

Thank you all!

[inetdog]

I really like the idea of using the auxiliary output from my CC (with the configuration of a low voltage disconnect) with a relay to power either:

1- A DC to DC battery trickle charger as Bala suggested (do you know where I can find one of those and how expansive they are?)
So, as I mentioned above, I really like the idea of to DC to DC charger via a CC aux. output powered relay. Just leaves me with the task of finding one!
or ....

...are you suggesting that I could simply hook up another PWM CC in parallel to my Outback and use this small CC to charge my battery.

Thank you all!

A DC to DC charger configured the way you envision it will have to have voltage boost capability, since there will be times when the output voltage will have to be higher than the input voltage, but not by very much. That will make it harder to find one.

You do not want to hook up even a small PWM controller to the same panels that are feeding your main CC or GTI. The PWM controller's pulsed current drain, trying to pull the panel voltage down to the battery voltage plus a volt or two during each pulse, will completely mess up the MPPT circuitry of the Outback and will cost you a good fraction of your total power. A lot more power than actually goes into the aux battery. Better to use a small inverter driving a small smart charger instead.

[SDC]

Well, looks like we have a winner!

Thank you to all of you for the inputs. It is well appreciated.

I will stick with the small inverter/small smart charger idea. Since I already own these two items, it appears to me as the cheapest, most efficient (timely speaking!) and safest solution (a paralleled PWM CC seams like something I might get trouble from...) . I already ordered a relay and will hook it up to my CC aux. output and set the low voltage disconnect so that it won't drain my bank, even though it appears almost impossible to me that it could happen.

Thanks again! I can't wait to see the temperature drop this winter and enjoy easy ATV starts with tip top shape battery!