Charging batteries from multiple sources?

System

Hi, new here, but have been working on designing a somewhat compact, semi-mobile solar power station for a while now. I'm seeking advice on a problem I recently ran into, which I'm sure many have already (though I can't seem to find a solid answer). Here it goes.

The problem is that I would like to be able to charge batteries with the PV panels whenever I can and as much as I can, but I would like to charge from an external source when I can't (eg., 12VDC from a vehicle). The question is, how can I charge batteries using two sources without damaging any part of the system? Here's a diagram of the problem:

Cariboocoot

Re: Charging batteries from multiple sources?

Welcome to the forum.

No worries; charging from multiple sources is done all the time. There are very few instances where any trouble comes up. One is using an automotive electrical system to charge deep cycles, because the vehicle system isn't design to hit the Voltage are meet the profile deep cycles need. As a temporary "boost it up 'til something better comes along" method it's okay. Certainly better than not charging at all!

Most of us off-gridders leave the PV's connected all the time and when the sun doesn't shine supplement charge with a generator power charger, either stand-alone or built-in to the inverter. Best case scenario is with an integrated system like Outback where the solar charge controller and the inverter/charger share data about set points and battery status.

You can have as many regulated charge sources attached to your batteries as you need.

System2

Re: Charging batteries from multiple sources?

Thanks a lot for the reply - I really appreciate it.

So, if I instead attached a Xantrex Freedom HF1800 inverter that could charge batteries off of shore power, then had a 12 VDC Cig Lighter charger (eg., Battery Tender), I could connect all three and the batteries wouldn't overcharge, due to the overcharge protection built into each device (charge controller, inverter/charger, and the Battery Tender)?

I think the Outback system is a little bigger/pricier than I had intended, so do you have any recommendations for a 12 VDC (cig lighter) battery charger that I could put in this system?

Will I need to put a diode in the line between the battery and DC charger?

Cariboocoot

Re: Charging batteries from multiple sources?

There should not be any need for blocking diodes on any of the devices, as they are all DC and should have rectified outputs (in other words, there's already a diode in the circuitry somewhere or else you don't get the DC output). How 'cleanly' they're rectified is another matter. You can always check for "leakage" (reverse current flow) with a particular device turned off and look for current flow back to it with a DC clamp meter.

Too technical, eh? How about "see if you have any problems, then disconnect each item in turn 'til the problem goes away"?

One thing that needs to be pointed out is that whichever device has the highest charge Voltage will take precedence. So if the Xantrex is designed to charge deep cycles to 14.4 Volts and the Battery Tender only "floats" a battery at 13.8 the system will be brought up to 14.4 by the Xantrex, regardless of other input. For the most part, it all works as long as none of the charge sources exceed the battery's needs.

Which is the next question: how much battery do you need to charge? And are you looking to be able to completely recharge it from solar, or just maintain?

Warning: it's only going to get more technical from here on.

System2

Re: Charging batteries from multiple sources?

Cariboocoot, thanks for the great info. I've been trolling forums for a while now and was able to dig up that I need to get a 'battery isolator' if I'm going to charge from the alternator. My only unhappy feeling about this option is that I was hoping to have the vehicle be independent of the 'house'. In other words, I was hoping to not have to chop the stock wires in the vehicle to charge the batteries from the alternator.

The size of the system hasn't been totally hashed out yet. So far, I think I'm going to be running a refrigerator during the summer and likely a small furnace in the Winter. My ultimate, though perhaps unrealistic goal is to charge completely from solar, even in the Winter. That's the part I'm worried about, though. I doubt that I will be able to rely 100% on solar even with the best solar panels, so I'm planning to have the capability charge using multiple sources (solar, shore, and alternator). Granted, the fridge will likely only take 0.7-3.0A when running, and the furnace is rated at 4.4A when running, that's quite a bit of power on those 16-hour long nights and the days when the sun sits near to the horizon around noon.

ATM, I estimate the battery bank to be about 200 Ah, and I may afford up to three amorphous panels on the roof (~200 W STC/each).

I'm sure there are some holes in this plan, so that's why I'm building in contingencies.

ggunn

Re: Charging batteries from multiple sources?

thevanman wrote: »

Cariboocoot, thanks for the great info. I've been trolling forums for a while now...

When you say you are trolling in forums, most folks will interpret that differently than I think you mean it.

Cariboocoot

Re: Charging batteries from multiple sources?

Primitive battery isolator: a switch. You just have to remember to turn it off when not in use.
The complicated ones are just automatic switches: they hook up the 'house' battery to the vehicle system when the engine is running, and disconnect when it's not.

Refrigerators are mean power consumers. The small ones aren't any better than the large ones really, especially in terms of Watts used for cubic foot of storage. Propane can be your friend there.

Furnace fans can also be nasty.

What you want to do is get the whole camper system worked out first, then see how much solar you can add to supplement charging. 200 Amp hours of battery recharged from solar only will demand 400 Watts of panel, more or less - and usually more. That for up to about 1 kW hour of power. Dismal, isn't it?

mike95490

Re: Charging batteries from multiple sources?

thevanman wrote: »

...ATM, I estimate the battery bank to be about 200 Ah, and I may afford up to three amorphous panels on the roof (~200 W STC/each). ...

amorphous - strange choice, how did you arrive at them ?

niel

Re: Charging batteries from multiple sources?

to add to mike's inquiry, i am not aware of a 200w amorphous panel and if there is a such animal it has got to be huge because they are about 1/2 the efficiency of the standard pv types. i know that there is a hybrid type from sanyo. did you mean them?

Shadowcatcher

Re: Charging batteries from multiple sources?

There are very power efficient options for camper use. I m using an Espar diesel heater which uses 21A for a couple of seconds during start up and drops to 1.8A on high and .7A on low, all LED lights, car stereo...
Propane RV furnaces are power hungry which is why I avoided them.
There is also a self (diesel) AC unit used in cooling truck cabs (expensive and cheaper to run the gen set)

uk_bryan

I have a similar question, I have a converted sprinter/ camper. I use two 125a/h leisure batteries. My charging system for the past few years has been a std battery charger when hooked up and a relay from the van battery when moving. The two never interfere with each other because I'm never hooked up to the mains when driving and never driving when hooked up. Simple really, but I am planning a trip around Europe and have decided to fit some solar to supliment the charging and reduce the need to use camp sites as much. 

My my worry is that when moving or when hooked up the solar charger will see the current coming from the relay and think the battery is more charged than it is and I will never really get fully topped off.

im going to fit a 160w solar panel and I've updated the charge controller to a 20amp mppt if that's relevant. There no point angling the panel as I have no idea what direction I will be facing at any time so it's going to be pretty much level. 


Thanks,


bryan. 

Estragon

It's not the current that's the issue, it's the voltage. If the alternator gets the battery voltage higher than the absorb setpoint on the charge controller, there will be no current from the panel. If the controller absorb voltage is higher, it will supply current as per its programming and the battery state of charge.