2 Qs re: thin film & battery parallel

**rama** · November 10th, 2010, 2:20 PST

Hi All,
Helping a friend setting up a project and got a couple of questions -

1. he's getting a good deal on thin film modules. while i recommended him using crystalline modules because thin films have very high voltage and complicates system design, not to mention additional system cost on rack support etc, he really loves the look of it. anyway, the 100W amorphous module has the following spec -

Voc=141V(initial)/138V(stabilized)
Isc=1.30A(initial)/1.24A(stabilized)
Vmp=111V(initial)/103V(stabilized)
Imp=1.19A(initial)/1.00A(stabilized)
Tc(voc)=-0.34%/'C

The system will use 48V battery and we got this MPPT charge controller that operates from 16V~112V with maximum 140VDC input. Now I know that this is drawing a bit too close to limit to be using this controller, but I don't have a lot of experience with thin films (esp the change in performance over time) and since our weather is pretty warm most of the time (always >25'C), I'm not worried about the voltage spike seen in cold weathers. What do you guys think, is this a good idea?

2. "Batteries should not be connected in more than 3 parallels". I was told this by a salesperson of batteries but I don't really buy it. I know connecting parallels can be tricky in terms of having the same (or similar) distance from each battery paralleled to point of output, so each battery can be drained more or less the same, but why the magic number 3?

As usual thanks for any help in advance - you guys are great (esp. BB!)

**stephendv** · November 10th, 2010, 3:49 PST

The initial Voc of the panels is above 140V. You will fry the charge controller. There is no place on earth that is always warmer than 25C, so even the stabilised Voc will fry the controller.

From my point of view, you only have 3 options:
1. Wait for the midnite classic to be released.
2. Go for the Apollo solar high voltage controller
3. Go for an AC side hookup using grid tied inverters and either a Xantrex XW or SMA sunny island on the battery side (and then order 20% more panels to make up for the efficiency losses).

**niel** · November 10th, 2010, 10:42 PST

it seems like a rock and a hard place with either too low or too high of a vmp, but there may be another solution that we can add to stephendv's list and we can call it #4. i am assuming there are 2 of these thin films in series right now so parallel them instead. i know the vmp is now low, but if you get another pv that has an imp of about the same of the 2 thin films paralleled then it could be placed in series with those 2 pvs paralleled. this gives the 1.19a x 2 for 2.38a from the thin films. putting say this 40w solartech spm040p in series with its 2.35a imp rating will work and this would give a vmp of 55.5v + 17.3v for 72.8v. voc will look like 70.5v + 21.8v = 92.3v. and this will put all specs within the parameters of the controller.
http://store.solar-electric.com/spm040.html
2 things to note is you need to mount the new pv with the other 2 and they will be differing in size and the increase in current may necessitate a larger wire and/or shorter run distance if the present one is not going to keep the voltage drop percentage under 3% with under 2% more preferred.

**rama** · November 11th, 2010, 0:52 PST

thanks stephendv, niel,

actually that's the voltage/current spec for 1 thin film module, so anyway I look at it I agree it's too risky. The charge controller has actually up to 150V of protection but will disable at 140V, so that just won't work.

manufacturer of the thin film module said it takes 3~6 months to stabilize, so i think we'll just find another mppt for that. other than the ones suggested, we came across another mppt with max Voc for 150V and operating range is 40~120V. given this thin film the only possible config would be parallel all the way, anyway, will give it a shot...

any thoughts on Q2? are there any other challenges (other than the one I mentioned) in having more than 3 parallels of batteries?

tks.