charging 12 volt battery from 24 volt panel?

James · June 2014

Hello folks.

I should know the answer to this question, but please help me out here.
i'm planning on putting together a small trailer to be used as a light duty portable power source.

It will have a small propane fueled generator and a small PV array sized about 400 watts.
Since I already have the generator, a 12 volt 2K watt inverter, and 12 volt agm batteries, all i need to buy is the PV panels and charge controller.

Now, here is my question;
can a 12 battery be charged by 24 volt PV panels, using a MPPT controller?

I'm finding the 12 volt PV panels cost way more per watt than 24 volt panels.
Is there a controller that will work this way or am I not thinking this out correctly?

If this confiration will work, would I be wasting available wattage generated by the panels?
thank you

inetdog · June 2014

Re: charging 12 volt battery from 24 volt panel?

James wrote: »

...
...
Now, here is my question;
can a 12 battery be charged by 24 volt PV panels, using a MPPT controller?

I'm finding the 12 volt PV panels cost way more per watt than 24 volt panels.
Is there a controller that will work this way or am I not thinking this out correctly?

If this confiration will work, would I be wasting available wattage generated by the panels?
thank you

You have found the strongest argument for using an MPPT controller. Grid tie panels with Vmp even higher than needed for a 24 volt battery are even cheaper than nominal 24V battery charging panels and putting panels in series reduces wiring costs.
You will not be wasting any panel power except to the extent that the conversion efficiency of the MPPT CC goes down slightly as the ratio of panel to battery voltage increases.

Cariboocoot · June 2014

Re: charging 12 volt battery from 24 volt panel?

Yes a 12 Volt battery can be charged from '24 Volt' panels using an MPPT type controller.

Wait 'til you see the price of said controller; the 'more expensive 12 Volt panels' suddenly look cheap.

The flipping point is around 400 Watts: that's when GT style panels @ $1 per Watt + MPPT controller become more economical than 'standard' panels & PWM.

BB. · June 2014

Re: charging 12 volt battery from 24 volt panel?

James wrote: »

Now, here is my question;
can a 12 battery be charged by 24 volt PV panels, using a MPPT controller?

Perfect fit--No problems (assuming size of array+controller are within specifications).

I'm finding the 12 volt PV panels cost way more per watt than 24 volt panels.
Is there a controller that will work this way or am I not thinking this out correctly?

The $64,000 question... "Cheap" GT panels + expensive MPPT charge controller. Or "Expensive" "12 volt" panels and cheap PWM controller.

Do a couple paper designs and see what works out best for you (MPPT controllers are going to have more options/programbility/options than the less expensive PWM controllers).

Take a look at these MPPT units and see if any are of interest to you:

http://www.roguepowertech.com/products/controllers.htm
MidNite Solar The Kid Charge Controllers and Accessories
Morningstar SunSaver 15 Amp MPPT Solar Charge Controller
Morningstar TriStar 30 amp MPPT Solar Charge Controller

If this confiration will work, would I be wasting available wattage generated by the panels?
thank you

MPPT controllers will work fine and efficiently... Just depends if MPPT controllers are "efficient enough" for your wallet.

Other than the ability to use "GT Vmp rated" panels (>>Vmp~17.5 volts for 12 volt battery bank), they do not have any special to offer a small/self contained system over PWM (over basic functionality).

-Bill

jcheil · June 2014

Re: charging 12 volt battery from 24 volt panel?

Go with the MPPT controller. It will give you more flexibility in the future when you expand (and you WILL expand - it's addicting).

James · June 2014

Re: charging 12 volt battery from 24 volt panel?

thanks folks, and yes i will expand, but not the trailer. with the lower panel costs these days, it looks inviting.
But it still hurts a littlke when I think of what we were all paying 10 years ago!

TucsonAZ · June 2014

Re: charging 12 volt battery from 24 volt panel?

BB. wrote: »

Take a look at these MPPT units and see if any are of interest to you:

http://www.roguepowertech.com/products/controllers.htm

MPPT controllers will work fine and efficiently... Just depends if MPPT controllers are "efficient enough" for your wallet.

-Bill

With 400 watts I would go with the above cc and the 24v panels. I will quickly do the math based on the research I've just done.

400w x $1.15 (cheapest I've found per watt based on Renogy panels after shipping) = $460
Quality PWM = $150

Total for your 400w not accounting for the lost power due to the PWM is $610 or $1.52/watt

460w x $0.69 (I just bought either 230w Solarworld panels at this price) = $318
Rogue MPPT = $250 (I just spoke with Marc, great guy to deal with)

Total for your 460w is $568 or $1.18/watt

That's a no brainer to me and will leave you with a far better system in the end.

elesaver · June 2014

Re: charging 12 volt battery from 24 volt panel?

460w x $0.69 (I just bought either 230w Solarworld panels at this price) = $318
Rogue MPPT = $250 (I just spoke with Marc, great guy to deal with)

Total for your 460w is $568 or $1.18/watt

If you're able, would you post the site for solarworld panels at this price? Thanks.

BB. · June 2014

Re: charging 12 volt battery from 24 volt panel?

Our host has Solarworld at even less (currently ~$1.03 to $1.10 per Watt--None of us moderators work for Northern Arizona Wind & Sun, we are just volunteers here):

http://www.solar-electric.com/hiposopa.html

However--make sure you have the costs delivered to your front door (o truck dock/local store)--Taxes, shipping, insurance, and handling for solar panels (especially for those >~140 Watts, and less than full pallets) can be eye opening.

-Bill

Cariboocoot · June 2014

Re: charging 12 volt battery from 24 volt panel?

Bill;

The panel only price was $0.69 per Watt, which is less than NAWS current price. The $1.18 per Watt was total for the array with the Rogue controller.

BB. · June 2014

Re: charging 12 volt battery from 24 volt panel?

Sorry--I missed that.

Anyway, watch shipping charges--Can easily add 50% to the panel price when buying less than a pallet load at a time.

-Bill

elesaver · June 2014

Re: charging 12 volt battery from 24 volt panel?

The panel only price was $0.69 per Watt, which is less than NAWS current price.

I was looking for the site with this price/watt on panels. I had checked NAWS first since I'd like to buy from them since they are kind enough to sponsor this forum.

jimindenver · June 2014

Re: charging 12 volt battery from 24 volt panel?

TucsonAZ wrote: »

t

Rogue MPPT = $250 (I just spoke with Marc, great guy to deal with)

Where did you find the Rogue for $250? The cheapest I can find the 30a version is $350.

I am still using the $100 Eco-worthy 20a MPPT controller I did a thread on here a few years back. It's still going and I bought a second to run a pair of 245w panels for 32a+ into one bank. The reason I'm considering upgrading is the need for RTS and voltage sense for mounting on the RV.

TucsonAZ · June 2014

Re: charging 12 volt battery from 24 volt panel?

elesaver wrote: »

If you're able, would you post the site for solarworld panels at this price? Thanks.

I did a ton of research on this, made a lot of calls and found a supply house locally that had the 230w panels as left over stock that got lost in their inventory once people started using the 250w and now 300w panels. They had 16 of them, of which I bought 8 and they'd joked about them having been there for eternity. I think their cost was actually $0.90 a watt back in the day and the sales manager had to get approval to sell them to me at $0.69 a watt but they were happy to have some of them gone. They won't ship but for anybody local and interested feel free to PM me for the contact info.

jimindenver wrote: »

Where did you find the Rogue for $250? The cheapest I can find the 30a version is $350.

I was talking about the 2024, he said 400 watts so I just used two of my 230 watt panels as a reference point and the 2024 should be fine with 460 watts at 24v, based on the specs of my panels anyway.

jimindenver · June 2014

Re: charging 12 volt battery from 24 volt panel?

Ok now I understand. My 20 amp controller is the same, 300w @ 12v and 600w @ 24v. I'd need the 30a for my panels flat mounted.

chelliet · May 2017

Hello,
I'm interested in buying 3- 320w PV panels, 24v. (Higher watts, lower prices) with a mttp controller. I also want to add a wind turbine 400/500/600w 12/24v. Is this possible? Approx. 1.5kw ? I plan on going full time rv living

My RV is currently running a 12-volt system. Microwave, coffee pot, 3way frig, Coleman roof ac. Full bath, kitchen, electric bed. TV, DVD, computer, phone and charging.

My Generac propane generator is 30 amps.

Inverter is a Heart; Input-12v dc, 120 amps max. Output- 120v AC 60 hertz, 1200 watts, duty cycle 15 minutes on min., 22 minutes at 750 Watts. 750 watts continuous.

Parallax converter charger. Series 7400. Input. 105-130 vac, 60 hz, 975 watts. Output 13.6vd , 55 amps.
(Includes charging and load)

I currently have 2 - 12v batteries but want to add 2 more.

Based on this information, can you confirm if this is will work. Also, can you suggest which charge controller that will work with wind and solar? I'm definitely interested in allowing for more space to upgrade later on. Therefore a decent charge controller is what I'm looking for at a reasonable price that would occur of course work appropriately.
Thanks ...Michele

littleharbor2 · May 2017

Chilliet, welcome to the forum.
Don't waste your money on any wind turbine. Their proper performance requires they be mounted 30 - 40 feet above any obstacles within hundreds of feet away, and be permanently mounted. That can't be done with a mobile RV. If you did mount it attached to your RV the vibration/resonance would drive you mad.
Run your fridge on propane or shore power. You will need, at a very minimum, 4 new healthy 6 volt golf cart batteries. You would need to be very frugal with your power consumption to live full time with this minimum bank. If you can find room , larger capacity, i.e.; L-16 type batteries would serve you better. Don't mix old and new batteries, you'll be replacing ALL of them sooner than you'd think.

jonr · May 2017

IMO, keeping functional old batteries in place as a parallel string with new batteries and charging the combination based on the state of the new batteries has a positive (less DOD), not negative effect on the new batteries. Even better if you measure balance and adjust it with cable lengths (offsetting the difference in internal resistance).

mcgivor · May 2017

jonr said:

IMO, keeping functional old batteries in place as a parallel string with new batteries and charging the combination based on the state of the new batteries has a positive (less DOD), not negative effect on the new batteries. Even better if you measure balance and adjust it with cable lengths.

Not exactly following you on that statement, connecting new and old batteries in parallel being a positive effect, kind of goes against most scientific evidence to the contrary, the general consensus is that the new battery will be drawn down to the level of the old and suffer a premature demise.

jonr · May 2017

OK, show me this evidence. When would the old be drawing down the new?

mcgivor · May 2017

jonr said:

OK, show me this evidence. When would the old be drawing down the new?

Sorry was thinking in terms of series, did some research and it appears you are correct, with the exception of differing cable lengths, at least according to the information provided in attached Pdf. Combining used and new batteries in parallel would be similar to using ones of different capacity in parallel.

mike95490 · May 2017

As batteries age, they loose capacity and their internal resistance increases.

What that means:
In Series, the Old batteries deplete first and can then be reverse charged, which will kill them in a day or two,
in Parallel, the New batteries, with their lower IR, do much more than 50% of the work, and are thus aged much faster till they are as bad as the original batteries, THEN they start to share the loads. This happens over a period of time, 4-10 weeks.

jonr · May 2017

Why does this new battery that is sharing some load with the old battery (even on day 1), age faster than a new battery that has nothing to share load with?

mcgivor · May 2017

This is going off on a tangent from the original subject, perhaps it's best to keep on track with the original question, all input considered, a seperate discussion would be benificial to all with questions related to mixing old and new batteries used together, but I'm sure this is not a new topic.

littleharbor2 · May 2017

Being as the original post is almost 3 years old, and Chilliet hadn't started ca new post I don't think we're straying too far off tangent.
Jonr, Part of the problem lies with old batteries have reduced capacity. Adding new batteries to them, with their larger capacity will cause the new batteries to be undercharged as the reduced capacity batteries will be fully charged sooner, leaving the new batteries not getting fully charged. The cumulative effect as time goes on is detrimental to the new batteries which for the most part never get fully charged. People mix new and old batteries all the time. That doesn't make it right and more than likely these mixed battery banks aren't going to get proper care and maintenance anyway. Personally I won't do it, especially to save a few bucks on a fresh RV design.

mcgivor · May 2017

Good catch, didn't realize the post was so old, the welcome to the forum in post#17 threw me off, wish posters would start a new thread instead of tethering to an old post.....rant over.

jonr · May 2017

As I said, "charging the combination based on the state of the new batteries" - so they get fully charged. And of course both parallel strings always see the same voltage.

littleharbor2 · May 2017

jonr said:

As I said, "charging the combination based on the state of the new batteries" - so they get fully charged. And of course both parallel strings always see the same voltage.

Is this something you do manually? How do you monitor the differences? The only true way is with a hydrometer. Then do you disconnect the old string while the new string is finishing bulk charging, then reconnect both for absorb, again for float? Sounds like a lot of unnecessary work for the cost of a couple more batteries. Meanwhile I'd rather be out exploring instead of messing around with my batteries. Each to their own I guess.

jonr · May 2017

You should always adjust charging parameters based on hydrometer results (ie, that is what I was referring to with "state"). No disconnecting needed.

charging 12 volt battery from 24 volt panel?

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