Help me build my solar system!

[4x4junkie]

Could be ok but my house system only is 8,260 watts. But it supports filtering a 60,000 gallon pool 9-7pm and a 2800sqft house with a 7kwh plugin hybrid just fine. ;-)

Yeah, that is precisely why I made light of that, even a large household residential system is a small fraction of that amount.


Perhaps a brief explanation of how "C" works in order here...

"C" = Capacity of the battery in amp-hours.
A battery's "C-rating" is how many times it's Ah capacity in amps the battery is capable of handling at maximum.
So for example: A 100Ah battery has a charge current rating of 5C (sometimes seen on high-end AGM units), this means you can charge it at up to 500A without harm to the battery. This does NOT mean any charge source hooked to it (such as solar) is going to charge it at that rate, it only indicates the maximum charge-rate that is safe for the battery.

So if one has 280 watts of solar on their 12V system, that 280W of solar will produce approximately 20 amps of current going into the battery at high-noon (at approx 14V). If the battery being charged has a total capacity of 200Ah, then that 20A current would be expressed as a "Charge-Rate" of 0.1C, or C/10 (one tenth the Ah capacity in amps).

At 0.1C charge rate, a lead-acid battery bank should be around 80-90% SoC when maximum absorptive charging voltage is reached.
SoC however will be less if max absorptive voltage is reached while at a higher charge rate (such as that coming from your alternator).

This is why I specifically stated in my earlier post "If you know how to interpret (a voltmeter's) readings...". It's quite easy to have a reasonably accurate idea of your battery's charge status if you have some familiarity with it & your system. Certainly I've never caught myself running out of juice because my voltmeter failed to give me any clues beforehand.

What's more interesting here is any battery with a 25C rating would have to be a lithium type (that's 2500A for a 100Ah unit), so to compare that to the charging (voltage) characteristics of a lead-acid unit is apples & oranges.

 

[loonwheeler]

There is a possibility we are getting units mixed up here. There is a difference between "C" (coulomb) and "°C" (degrees Celsius). Maybe Joe917 meant that the voltage during bulk charging is set at 14.8 V when the cell temperature is at 25 °C - not that he bulk charges the battery at 25C (25 X the C20 capacity of the battery). May seem obvious to most but thought I would point it out.

 

[4x4junkie]

There is a possibility we are getting units mixed up here. There is a difference between "C" (coulomb) and "°C" (degrees Celsius). Maybe Joe917 meant that the voltage during bulk charging is set at 14.8 V when the cell temperature is at 25 °C - not that he bulk charges the battery at 25C (25 X the C20 capacity of the battery). May seem obvious to most but thought I would point it out.

That little "°" symbol you noted indeed does completely & entirely change the meaning of C (an asterisk (*) can be substituted for the ° symbol too if one isn't able to find or use it on their device). I would've thought my 20 amps into a 200Ah bank example for 0.1C would've made it clear I wasn't referring to anything about temperature.

If this wasn't the case, then I do apologize.


"C" in my reference was for Capacity (of the battery in Ah)

 

[birdman86]

*Bump* with a bit of a side bar

I think I figured out what I need for solar but for the price I'm not ready to make that purchase just yet and with just a couple weeks until I plan on hitting the road I'm looking at setting up my battery bank and just using an isolator to charge them off my alternator. I figure it should be enough to run the roof fan and house lights for a weekend, everything else like fridge, TV, device charging can come later. So before I buy an isolator...

- Looking at this isolator that I basically found by searching "battery isolator" on Amazon.
- I should need up to 60Ah/day (very generous estimate). Double that for the whole 50% charge rule, and for easy round numbers, I'd need at least 100Ah of capacity assuming I drive every day.
- I don't know much about alternators - how fast would this isolator charge my battery bank? If I'm consuming 50-60Ah/day how long would I have to run the engine to charge the aux back up?
- Am I sacrificing anything by going this route? Could I risk the alternator being unable to keep up with keeping my starter battery in prime shape? The whole reason for solar was because I don't ever want to risk a dead starter battery.
- Should I buy a battery monitor or something for a setup like this? Or is there some lucky difference where alternators regulate themselves?
- Installing these is as simple as connecting the starter battery to the battery bank in parallel, but putting the isolator inline with one of the parallel connectors? Does it matter if its the pos or neg?
- Any reason not to go with dual 6V battery bank in series, and connecting that in parallel to the starter as mentioned above? If no I can just get the solar batteries I'm planning for, if yes I'll just find something cheap that has the Ah I need for the time being.
- Going this route, will I need to ground the battery bank to the frame/starter battery, or is the isolator good enough to keep things completely independent from the vehicle?

 

[dwh]

*Bump* with a bit of a side bar

I think I figured out what I need for solar but for the price I'm not ready to make that purchase just yet and with just a couple weeks until I plan on hitting the road I'm looking at setting up my battery bank and just using an isolator to charge them off my alternator. I figure it should be enough to run the roof fan and house lights for a weekend, everything else like fridge, TV, device charging can come later. So before I buy an isolator...

- Looking at this isolator that I basically found by searching "battery isolator" on Amazon.

Someone around here years ago said they work at a cop shop and they use Stingers in their cop cars. All it is is a relay. You rig it as a "split-charge relay":

http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50

Personally, if I was starting from scratch, I'd go with a relay capable of handling the full output of the alternator, and use wire big enough to handle that. But yea, the Stinger will work. My truck has a 100a alternator and an 80a relay like in the link above and it's worked without a hitch for years.

- I should need up to 60Ah/day (very generous estimate). Double that for the whole 50% charge rule, and for easy round numbers, I'd need at least 100Ah of capacity assuming I drive every day.
- I don't know much about alternators - how fast would this isolator charge my battery bank? If I'm consuming 50-60Ah/day how long would I have to run the engine to charge the aux back up?

If you have a new type computer controlled charging system, then it won't be too bad. If not, then you can expect to drive for a few hours to get the battery to 80% or possibly 90% (depends on the voltage that your voltage regulator is set to) and another 12-24 hours of drive time to finally reach 100% charged. If you're just idling...figure...oh...a week to get back to 100%. (Just kidding...it won't be a week...probably only be a couple of days.)

- Am I sacrificing anything by going this route?

Sacrificing multi-stage (bulk/absorb) charging.

Could I risk the alternator being unable to keep up with keeping my starter battery in prime shape? The whole reason for solar was because I don't ever want to risk a dead starter battery.

Starting the truck probably uses around 1/5 of one amp*hour. You could keep up with that using an alternator from a model airplane.

- Should I buy a battery monitor or something for a setup like this?

For a simple split-charge relay? Nah. I just use a voltage meter. After a while, you get a feel for how things are going.

Or is there some lucky difference where alternators regulate themselves?

If alternators regulated themselves, they wouldn't need voltage regulators. The voltage regulator just turns the alternator on/off to keep the voltage where it's supposed to be. (Unless you have a modern computer controlled charging system, then it's got some additional bells and whistles.)

- Installing these is as simple as connecting the starter battery to the battery bank in parallel, but putting the isolator inline with one of the parallel connectors? Does it matter if its the pos or neg?

Theoretically it doesn't matter. In the real world, if you put it in the neg then Santa moves your name to the naughty list and you have to start speaking Queen's English.

But seriously...if you put it in the neg, then how in the hell are your aux loads gonna work when your aux battery has the neg cable disconnected? You ground the battery permanently and use the relay to tie the aux battery to the charging system when the engine is running, and disconnect it when the engine isn't running.

- Any reason not to go with dual 6V battery bank in series, and connecting that in parallel to the starter as mentioned above? If no I can just get the solar batteries I'm planning for, if yes I'll just find something cheap that has the Ah I need for the time being.

No reason. 12 volts is 12 volts.

- Going this route, will I need to ground the battery bank to the frame/starter battery, or is the isolator good enough to keep things completely independent from the vehicle?

The "isolator" doesn't "isolate" the second battery from the vehicle. It only "isolates" the two batteries whenever the engine is off, so that your loads on the aux battery don't draw down the engine battery. So yes, you have to ground the aux battery to the truck somehow. Frame works, but for a really long run (20' or more) I'd run a dedicated ground alongside the hot back to the engine battery.

 

[birdman86]

Thanks for the link, very helpful...all 66 pages haha

Pardon my many many basic questions, just one more (for now...)

If I buy a cheap dual volt/ammeter just for a basic monitoring of the aux battery, how do I wire it up? All the diagrams have it wired in-line with the battery cable, so I'd put it between the alternator and the aux battery. But those cables are 4awg and the meter looks like it's around 12awg maybe. Would I just run a fused 12awg line alongside the hot 4awg or will there be issues with having "two lanes" going alt/relay -> aux + post? Most definitely over thinking this I bet...

For what difference it makes the aux battery bank would be in the back, behind the rear wheel well so that's a pretty long run.

 

[4x4junkie]

Thanks for the link, very helpful...all 66 pages haha

Pardon my many many basic questions, just one more (for now...)

If I buy a cheap dual volt/ammeter just for a basic monitoring of the aux battery, how do I wire it up? All the diagrams have it wired in-line with the battery cable, so I'd put it between the alternator and the aux battery. But those cables are 4awg and the meter looks like it's around 12awg maybe. Would I just run a fused 12awg line alongside the hot 4awg or will there be issues with having "two lanes" going alt/relay -> aux + post? Most definitely over thinking this I bet...

For what difference it makes the aux battery bank would be in the back, behind the rear wheel well so that's a pretty long run.

If you want to measure amps into the battery from your alternator, then you need a meter that uses an external shunt capable of whatever your alternator's max output is (probably around 100A). Such a shunt would go inline with the battery negative cable if you want to also observe amps going out of the battery (same as done with the shunt for a battery monitor).
Connecting an amp meter alongside a cable like you mention won't work.

 

[birdman86]

If you want to measure amps into the battery from your alternator, then you need a meter that uses an external shunt capable of whatever your alternator's max output is (probably around 100A). Such a shunt would go inline with the battery negative cable if you want to also observe amps going out of the battery (same as done with the shunt for a battery monitor).
Connecting an amp meter alongside a cable like you mention won't work.

Ahh crap, I threw it in my amazon order anyway because for $13 why not.

End of the day, do these meters really add value on a basic isolator setup like I'm looking at? I've basically sourced everything I need to match the first option on that DIY link posted above.

 

[goofballracer]

tons of great info here keep it up

 

[dwh]

When you know your gear, specs, loads...then you pretty much know your rig's status. Voltmeter is enough to confirm what you already know.

Same like oil - pressure gauge is fine...don t need a level gauge as well.

 

[4x4junkie]

Ahh crap, I threw it in my amazon order anyway because for $13 why not.

End of the day, do these meters really add value on a basic isolator setup like I'm looking at? I've basically sourced everything I need to match the first option on that DIY link posted above.

Yeah as dwh just added, the voltmeter side of that unit can be of use. The amp side (being only 10A) would probably be more limited as to what you can do with it.

Hopefully that unit has an adjustment potentiometer on the back of it, occasionally cheapie units like that can be off just a little bit (0.1-0.2V or so).