Solar panel help, 36volts to 12v to charge a battery

Thetruthking

Hi all, I eventually changed the panel and now am going to try and set up so it will charge a car battery for me to use power off the whole time!

I was givin a panel today its spec is:
Max Power (pmax) 230wp
Voltage at Pmax 29v
Current at Pmax 7.93A
Opwn-Current Voltage 36.83v
Short-Circuit Current 8.52A

I currently have a 20Amp solar charge controller but I can not connect this to it as the voltage is too high and the solar controller would just switch off to protect itself

I order this: http://www.ebay.ie/itm/351080171974?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
in the hope of lowering it to about 14 or 15 to put into the solar controller so I can charge the battery etc

Could anyone tell me if
1. if this would work
2. is their any other way of lowering the voltage to connect it to my solar charger controller
3. Whats the best thing I can do to get thos panel to charge a car battery that I use to power 12v lights etc?

Im just looking for an easy way to put this panel through the solar contrtoller (as it protects agenst overcharge etc) so I can charge the 12v battery to use lights and even an invertor etc


Thanks in advance everyone,

quentingargan

Afraid that won't work. Your solar panel is fine for charging a 12V battery, but what you need is a 12V MPPT charge controller. This will take in 29V at 7.9A and push about 16A at 13.5V into your batteries.

The trouble with the DC DC converter you have is that it will deliver 12V, but what you need to charge batteries is between 13.5 and 14V.

PM me if you want a link to a suitable charge controller. But if you google 20A MPPT charge controller 12V you should see something that will work.

Don't go for a charge controller that isn't MPPT with an MPPT voltage of up to 30V. Also, check that its VOC (open circuit voltage) isn't any lower than the VOC for your panel. Allow a few volts for low temperature because the VOC for a panel is stated at 25C whereas it rises with low temperature.

Thetruthking

Hi Thank you for the reply,
Yes my controller is MPPT but it says max is 17v for 12v battery and 30 for 24v battery but it auto switches off if it goes over 18v input when a 12v battery is being charged, (to protect it I guess) the DC DC converter is adjustable, I can set the output to anything from 0.8volts to 28volts, if I connect the panel to the DC - DC think I linked above and set the output to about 16 or 17v then connect it to the charge controller would that work?
thanks in advance

quentingargan

Thetruthking wrote: »

Hi Thank you for the reply,
Yes my controller is MPPT but it says max is 17v for 12v battery and 30 for 24v battery but it auto switches off if it goes over 18v input when a 12v battery is being charged, (to protect it I guess) the DC DC converter is adjustable, I can set the output to anything from 0.8volts to 28volts, if I connect the panel to the DC - DC think I linked above and set the output to about 16 or 17v then connect it to the charge controller would that work?
thanks in advance

That DC DC converter will probably not leave the module at its optimum voltage to operate, so you will lose a lot of efficiency. Then on the other side, it depends on whether your MPPT charge controller has the ability to boost voltage upwards. Some high quality MPPT charge controllers can take in 12V and put out 13.5V to charge the batteries, but most don't.

I think you would be better off putting your charge controller on eBay and buying one that is designed to do the job. There are units out there that can take in up to 60V and charge a 12V battery, while keeping the module at its optimum voltage.

quentingargan

That DC converter will not last it's dodgey Chinese junk.
MPPT is great for large arrays, but not worth the investment on a small rig imho.
PWM is all you need and PWM is more durable.

quentingargan wrote: »

That DC DC converter will probably not leave the module at its optimum voltage to operate, so you will lose a lot of efficiency.

From what I've seen MPPT loves regulated voltage compared to PWM chargers, it's way more effective, and by this I mean a further efficiency increase on the already apparent MPPT versus PWM unregulated supply comparison (minus the regulator losses so nul points unless you are using a PSU instead of a PV module (handy trick to make a cheap quality mains charger though))

quentingargan wrote: »

I think you would be better off putting your charge controller on eBay and buying one that is designed to do the job. There are units out there that can take in up to 60V and charge a 12V battery

+ 1

with a temperature sensor and separate voltage detection wires (eliminates inaccurate readings off load carrying conductors).

Solar controllers are just semi-intelligent buck(/boost) converters anyways. Less losses with one instead of two.

TriStars are fantastic PWM controllers.
I'd go MidNite Classic for MPPT.
Both will accept the voltages you are producing.

(No affiliation other than user, delete if necessary Mods. they're products not companies)

quentingargan

Agree, but if you use a PWM controller, it can only allow the maximum amps from the module to the battery. You are stuck with a mismatch between 30V on the module and 13.5V on the battery. The max current on the panel will be about 8 or 9 amps, so the most you will get out of the module with a PWM charge controller is about 110 watts. So in this instance, the OP either needs to go to 24V batteries (with PWM controller) or go for MPPT.

Ah interesting, I must have a look at that. I've seen 170W coming from one of your 150W modules through a PWM TriStar 45. I'd be more inclined to credit that to cloud edging on a cold day though.

Silly me I thought MPPT used feedback to balance the cell output of the array. MPPT explained.
The feedback theory didn't make any sense when confronted with a regulator in the middle but that was an experimental discovery and I didn't question the hows.

Thetruthking

Back to the OP questions

Thetruthking wrote: »

Could anyone tell me if
1. if this would work

Yes but it's messy. If you were to pursue this as a circuit configuration then get at least 2 regulators because they're the biggest liability in the plan (they generally do what they say but they're cheap components and fail mechanically more often than not while you are adjusting them. The one you listed is a buck/boost converter a buck converter may be cheaper and more efficient although the boost feature may enhance your harvest at dawn and dusk.
Overall the loses of the regulator maybe offset by the gain of the MPPT controller so there's effectively no system loss, but this can only be confirmed experimentally I expect.

Thetruthking wrote: »

2. is their any other way of lowering the voltage to connect it to my solar charger controller

Unless you get an ~18v module or controller that'll handle the voltage at the panel none that won't introduce system losses (resistors, orientation of panel away from solar incidence, partial shading (not recommended for cell longevity))

Thetruthking wrote: »

3. Whats the best thing I can do to get thos panel to charge a car battery that I use to power 12v lights etc?

Suitable temperature compensating MPPT controller (or swap the module again for one that matches your controller). Batteries as close as possible to the controller with cable gauged to less than 0.3% voltage drop to battery, DC MCB protected (lower impedance than a fuse and resettable...you will need to reset the controller from time to time). UV resistant cable less than 3% voltage drop panel to controller also DC MCB protected to protect the controller from OC voltage when you are resetting the controller, also useful for servicing the rig.
Then fuse or DC MCB protected lights wired from the battery or the charge controller's LVD terminals that are a nominal 12v but have a 10v > 16v input threshold or wider.
To charge the battery check the datasheet for the battery for bulk, absorption and float set-points and configure (if it's programmable) the charge controller to match these exactly. If it's a fixed algorithm controller then select one that matches the battery.
In my experience none of the fixed algorithm controllers are suitable for open lead acid, and most are only suitable for sealed batteries when set to the open lead acid set-points.
The entire industry seems paranoid about overcharge and prefers undercharging as an ironic solution.

SmartGauge wrote:

{Extract from:}

Obviously charging them exactly to 100% is preferable (with the occasional controlled overcharge in the case of wet cells) but this is rarely possible. If this is the case, err on the side of overcharging them rather than undercharging them. Continually overcharging by a certain amount might shorten a battery's life by say 20%. Continually undercharging the same battery by the same amount might shorten it by 90%.

[*Sorry Mod's I'm promoting the technical info. not the products...delete away*]


Last and most importantly get a good battery monitor that reads current as well as volts, and preferably gives a state of charge reading in % based on the current used and not voltage.

Sir Liamalot

Sir Liamalot wrote: »

From what I've seen MPPT loves regulated voltage compared to PWM chargers, it's way more effective, and by this I mean a further efficiency increase on the already apparent MPPT versus PWM unregulated supply comparison (minus the regulator losses so nul points unless you are using a PSU instead of a PV module (handy trick to make a cheap quality mains charger though))

Thought it best I amend this. It needs to be a current limited PSU or it will eventually let the magic smoke out.

....no particular reason that I'm mentioning this