Solar again

I've been banging this around in my head for so long I'm starting to hear rattling noises, but this time I mean it...I'm close to making the plunge and will call a few distributors next week, but thought I would present an idea here.

My plan is to build a hybrid system, one that will allow me to charge golf cart batteries from either a 100W solar panel or the truck's alternator. Furthermore, I don't plan to mount the panel on the truck. Instead it will be a remote unit on a stand of some sort that I plug into the truck using heavy duty quick connects. That way I don't have to worry about it getting damaged by low hanging branches and I can park in the shade, but have the panel in the sun (provided I can find such a place). Also, I'll be able to turn it throughout the day to get maximum efficiency out of the panel. The draw back is that the batteries will be discharging when I don't have the solar panel plugged in.

From my research on the internet, golf cart batteries can be drained as much as 50% with no harm done. However, when drained that much, they must be recharged slowly (no more that 20% of their Amp-Hour rating). The truck's alternator would probably charge them far to quickly.

The MPPT charge controller I'm think of getting is a smart charger and should work well with the golf cart batteries.

So, here is my idea. Connect the input circuit of the charge controller to the truck's electrical system. I would use a break-before-make switch to select either the solar panel, or the truck's system. Furthermore, I would incorporate a DC-DC converter to limit the current from the truck to something the charge controller can handle.

Does this sound like a reasonable design? Or, am I likely to fry components? What might I be missing? I'm starting to hear rattling again!

Did I understand you want to regulate the output your alt sends to the aux. bank with the solar charge controller? If so I was wondering the same thing and called a solar shop and they told me solar controllers aren't set up for that and apparently solar output is somehow different. I didn't get into too far since I haven't connected the aux. bank to my truck yet. Have you talked to the knowledgeable solar shops about that part yet? I'll be interested to hear if you find out something that works.

Did I understand you want to regulate the output your alt sends to the aux. bank with the solar charge controller?

Yes.

If so I was wondering the same thing and called a solar shop and they told me solar controllers aren't set up for that and apparently solar output is somehow different. I didn't get into too far since I haven't connected the aux. bank to my truck yet. Have you talked to the knowledgeable solar shops about that part yet? I'll be interested to hear if you find out something that works.

I haven't talked to any professionals yet. I came up with the idea last night. The solar shops aren't open on the weekend, so I'll be making some calls this week.

I'll let you know what I find out.

Solar Link

This site has some good information. Keep it simple, the fewer components you use the better.

http://www.windsun.com/

Thanks drtrat

I've garnered information from there and over a dozen other sites. Thanks for the link.

I assume you are wanting to regulate the alt charge to properly charge your batteries and not cook them? If so we're in the the same boat. With my agm's the alt. isn't the best profile for charging and definately not good for floating them. Right now I'm considering a volt gauge in the cab for monitoring them and when they are due for bulk charging connect them to the alt. via a solenoid and manually controlled switch (ignition hot -> switch -> solenoid) to give them a shot. Periodically disconnect the charge to see what the voltage of the batteries is at to get them 90% charged or so relatively quickly (ie something along the lines of 10-30amps). Then I was thinking of connecting a smaller inverter to my truck and plugging my 5amp smart charger into that to work on fully charging the batteries and floating them properly.

I wouldn't really bother with any of this for weekend voyages when I can come home a day or two later and just let my smart charger do all the work but longer trips that would require maintaining the batteries when shore power wouldn't ever be readily available.

However if the solar controller does end up having the potential to work AND you were going to get one anyways I'd definately pursue it.

First thing i would say is not to drain the batteries to 50% on a regular basis.

I'd bet that the truck alternator won't be putting out much current or voltage at the camper batteries. It would be interesting to see actual numbers.

I'd hook up the output of the solar charge controller to the battery and also the alternator output.

I don't see a reason to run the alternator output through the charge controller.

I had time to call a few people today.

If I understand things correctly, the solar charge controller expects to see voltage and amperage increase as the solar panel efficiency goes up (that is, it gets more direct sunlight, combined with cool temperatures). If the input is say 13.8 volts and 5 amps (69 watts) that is not the same to the controller as 15 volts at 4.6 amps (69 watts). So my idea was nixed by 2 different companies, but I got points for thinking.

An internet search for "DC input battery chargers" found a few companies that make 3 stage (bulk, equalization and float) chargers that take an input of 8-14 Volts.

Right now I'm considering a volt gauge in the cab for monitoring them and when they are due for bulk charging connect them to the alt. via a solenoid and manually controlled switch (ignition hot -> switch -> solenoid) to give them a shot. Periodically disconnect the charge to see what the voltage of the batteries is at to get them 90% charged or so relatively quickly (ie something along the lines of 10-30amps)

That's what one of the guys I talked to today does. He even turns it off going up hill when the batteries are low so that he has the extra horse power for moving his RV.

First thing i would say is not to drain the batteries to 50% on a regular basis.

I educated myself further. Batteries down to %80, not %50 is a much better way to go.


I'd bet that the truck alternator won't be putting out much current or voltage at the camper batteries. It would be interesting to see actual numbers.

My digital multimeter is only good to 10 amps DC. I would like to see real numbers too. The truck alternator without any regulation (provided the wire size is sufficient) should work well for bulk charging. It's the float charge that I'm thinking about (though I didn't understand it as well yesterday as I do today).

I'd hook up the output of the solar charge controller to the battery and also the alternator output.

The charge controller is designed to work in this configuration.

Thanks for your input. It helps a lot!

An internet search for "DC input battery chargers" found a few companies that make 3 stage (bulk, equalization and float) chargers that take an input of 8-14 Volts.


Promariner makes a "digital mobile" charger that does this but they run over $300: http://promariner.com/productFeature.php?ProductNum=05504

Power stream makes a smaller amp one for ~$150 but I haven't dug up the numbers on its charge profiles yet: http://powerstream.com/DCC.htm

I'm still wondering what downside there would be to float the batteries via an AC charger running off an inverter, aside from the obvious efficiency loss but I wouldn't think it a major issue when running a 5amp charger for instance.

I'm starting to lean toward this.

Promariner makes a "digital mobile" charger that does this but they run over $300: http://promariner.com/productFeature.php?ProductNum=05504

Power stream makes a smaller amp one for ~$150 but I haven't dug up the numbers on its charge profiles yet: http://powerstream.com/DCC.htm

I'm still wondering what downside there would be to float the batteries via an AC charger running off an inverter, aside from the obvious efficiency loss but I wouldn't think it a major issue when running a 5amp charger for instance.
Assuming the truck's battery(ies) are charged (which they should be if only used to start the truck), then I would think the alternator would have enough reserve capacity for an inverter powered smart charger.

My biggest concern would be the current draw (considering loss for running the inverter, then again the charger). Assuming the inverter converts 85% of it's input power to output, and the charger does the same, then 20 amps of input to the inverter equals 14.45 amps to the batteries.

20 x 0.85 x 0.85 = 14.45

My second biggest concern would be heat generation. The current that goes into the pipe, but doesn't come out of the pipe, is lost in heat.

20 Amps - 14.45amps = 5.55 Amps

5.55 Amps x 12 Volts = 66.6 Watts

That means (assuming my other assumptions are correct) that there is a 67 watt heater running some where in the truck or camper.

If one could manage the heat and power (which shouldn't be too much trouble considering heat sinks will be built into the 2 components), then it might be the least expensive, off the shelf solution.

I found the powerstream site, but their 12VDC input is only rated to charge up to a 100 AH battery. If I go with golf cart batteries in series, I'm looking at a 200 AH 12 volt battery. I suppose I could do like you suggest - bulk charge for a while and check the voltage - until they reach around 12.5 V, then switch to a smart charger to equalize charge and float charge the last 10%. However, I know I'm likely to forget to check the batteries

My biggest concern would be the current draw (considering loss for running the inverter, then again the charger). Assuming the inverter converts 85% of it's input power to output, and the charger does the same, then 20 amps of input to the inverter equals 14.45 amps to the batteries.


Yeah you'll be loosing power but its a trade off of cost and such. However I wouldn't necessarily try my idea for the bulk charge portion so I'd be scaling those numbers down to a 6amp type smart charger which I think would be a lot more do able and friendly on the situation.

If one could manage the heat and power (which shouldn't be too much trouble considering heat sinks will be built into the 2 components), then it might be the least expensive, off the shelf solution.

In my case I already have both the components on hand (smaller inverter and smaller smart charger) so I was just trying to rationalize out why not to do it rather than buy another expensive component to properly charge things.

I found the powerstream site, but their 12VDC input is only rated to charge up to a 100 AH battery. If I go with golf cart batteries in series, I'm looking at a 200 AH 12 volt battery. I suppose I could do like you suggest - bulk charge for a while and check the voltage - until they reach around 12.5 V, then switch to a smart charger to equalize charge and float charge the last 10%. However, I know I'm likely to forget to check the batteries

I'd think you would still be able to use the smaller charger it would just take longer, no? The memory issue I can't help with, lol.

By the way if you're running golf cart I think you can breath a bit easier on the charge profiles because even if you do overcharge (float too high) them I think you can just top them up with water. I my case I'm running dual 6V agm and once they vent there is no putting it back...