Panel voltage and best voltage for charging (2 8D) 12V batteries

pocketchange@copper.net

Greetings from a Old' new to Solar (and this forum)..

In light of a little knowledge being almost dangerous and putting a new system together, my question concerns the best way to wire up my panels.

I have 6 90W X 12V panels and was thinking going Series (pairs) would be a better option than running
a straight 12V system. I'm charging a pair of 8D batteries and need to maintain enough voltage
to keep the TriStar TS 45 doing its thing and it also being more economical to wire in series.

Hopefully I'm thinking in the right direction.. pc

Cariboocoot

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Welcome to the forum.

Well you're kind of thinking right in that you need sufficient Voltage from the panels to raise the batteries to their proper charging level.

But the panels probably already provide this as a "12 Volt" panel will usually have a Vmp (Voltage at maximum power) of 17-18. This provides enough "Voltage overhead" to allow for V-drop from operating temperature and wire loss.

What's more the TriStar comes is two versions: PWM and MPPT. With the former you do not want higher than the typical panel Vmp because it will be a waste of power: PWM controllers do not have the ability to down-convert higher Voltages into greater charge current. MPPT controller do, but even so the higher you go in Voltage on the input in respect to Voltage of the battery the lower the controller's efficiency. As a rule MPPT input nominal Voltage should not be more than 2X the nominal system Voltage (i.e. 24 Volts for a 12 Volt system).

Your panels total 540 Watts. On an MPPT controller and 12 Volt system you should expect a peak current of 34 Amps. On a PWM controller probably (90 Watts / 17.5 Vmp * 6 panels) 30 Amps. "8D" is a battery case size, not an Amp hour rating so we don't know if either current is sufficient for two of them.

Do you have the exact specs for the panels (Vmp, Imp, Voc, Isc) and make/model of the batteries?

pocketchange@copper.net

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

16.5 voltage, 5.4 Amps, Grey Gel Cell 8D, on a RV.

Cariboocoot

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

pocketchange@copper.net wrote: »

16.5 voltage, 5.4 Amps, Grey Gel Cell 8D, on a RV.

Well 16.5 Vmp can be low in some cases.
The batteries sound like Trojan 8D gel cells: http://www.trojanbattery.com/pdf/datasheets/8DGEL_Trojan_Data_Sheets.pdf
That's 225 Amp hours each, or 450 for the two in parallel. In that case 30 Amps would be a 6% peak charge rate. Note the specs on the charging too: Absorb maximum of 14.4 Volts. Gels are very sensitive to high Voltage - it will damage them quickly.

Now do you know which TriStar you have (PWM or MPPT)?

pocketchange@copper.net

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

TriStar TS 45 PWM

Gel-Tech 225 Amp Hrs., 475 Reserve Capacity

Cariboocoot

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

pocketchange@copper.net wrote: »

TriStar TS 45 PWM

Gel-Tech 225 Amp Hrs., 475 Reserve Capacity

With the PWM version of the TriStar you have no choice but to put all the panels in parallel. In doing so you should use a combiner box and have fuses or breakers on each one. The panels will produce (6 * 5.4) 32 Amps peak power or 7% rate, better than I originally expected.

pocketchange@copper.net

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Thank You for the help.

Would I be better off with an MPPT controller (I have not bought one as yet, trying to save a little money?)

Nice to know I'm headed in the right direction..

Cariboocoot

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

pocketchange@copper.net wrote: »

Thank You for the help.

Would I be better off with an MPPT controller (I have not bought one as yet, trying to save a little money?)

If you were to use an MPPT controller you'd have the panels as three parallel strings of two in series, so you'd still need combiner and fuses. The current gain would only be 2-3 Amps. Probably not worth the extra $266.

One other thing: you need to use 8 AWG wire from the combiner to the controller and then to batteries to handle the current. Hopefully the distance will be short and in combination with the large wire the V-drop should be minimal.

pocketchange@copper.net

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Whew!
Man I'm glad to hear that news..!

I've been scrounging around looking for the proper sized cable (and found the #8.)

I was going with #8 from the panels & same (maybe #6) from the combiner to the controller and 1/0 controller to batteries.

Next is the breakers/fuses and switch.

Cariboocoot

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

In terms of fuses, the panels themselves would have 10 Amp fuses (or breakers), one each. That is to protect against one of them being shorted and the others putting their combine current to it (5 panels * 5+ Amps = 25+ Amps which is more than one of them is likely able to handle).

You do not actually need a fuse/breaker between the combined panels and the charge controller input.

The controller output should have either a fuse or breaker capable of its full capacity on that wire size. With the TriStar 45 that works out to 50 Amps as the 8 AWG can handle that much continuously and the panels will be producing around 32 maximum. That protection is for in case the controller shorts and the battery feeds its immense current capacity into that short. If you want/need a disconnect here you could use a breaker instead. Make sure it is a DC rated breaker. Some examples: http://www.solar-electric.com/cipr1.html

pocketchange@copper.net

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Thanks Again..

I'm going to take your suggestions to Heart and will get back to you when time comes (shortly.) pc

inetdog

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Cariboocoot wrote: »

Make sure it is a DC rated breaker. Some examples: http://www.solar-electric.com/cipr1.html

I would say make sure that either the breaker or a fuse are DC rated. Fuses can have lower DC than AC voltage ratings too.
With DC rated breakers, you still have the potential problem that the method they use to break the arc is polarity sensitive. It will work when the current is flowing in one direction but not in the other. Result: You have a choice of installing it to break fault current from a failed CC back to the panels, driven from the battery voltage OR install it to allow you to use it to disconnect the panels while the sun is shining. A polarity sensitive DC breaker cannot do both.

vtmaps

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

inetdog wrote: »

With DC rated breakers, you still have the potential problem that the method they use to break the arc is polarity sensitive. It will work when the current is flowing in one direction but not in the other. Result: You have a choice of installing it to break fault current from a failed CC back to the panels, driven from the battery voltage OR install it to allow you to use it to disconnect the panels while the sun is shining. A polarity sensitive DC breaker cannot do both.

Except at Midnite solar. Their polarized breakers are installed as string switches, not as overcurrent protection devices. I think that they can get away with it because in a combiner box the maximum interrupted current is not very high... certainly not like a battery fault current.
None-the-less, if I am correct about the low interruption currents, I still don't understand why they would install them that way. The purpose of the NEC mandate on combiner breakers is to provide overcurrent protection.

--vtMaps

Vic

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

As Cariboocoot mentioned, Gel batteries are delicate. It is essential that you use a Battery Temperature Sensor on every charge source -- in your case, at least on the CC. And if you use another charger, there, too. Would suggest that you use the mid-range of suggested charge voltages. Also be certain that the Charge Controller can accommodate the correct Temperature Compensation. For sealed batteries, like Gels, Rested battery voltage is the best measure of State Of Charge. This rest period is usually a minimum of 3 hours. Overnight rests are even better. Rest IS when there has been NO CHARGE and NO LOADS for the stated period. This voltage measurement must be temperature compensated, and made with a known-accurate meter.

Use the exact data from the manufacturer of the batteries that you have for charge voltage, temp comp, and any limitation in the Maximum Charge Current. Some Gels are limited in this max current to about 5% of Capacity, some have no such limitation, within reason. Opinions, Good Luck, Vic

inetdog

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

vtmaps wrote: »

The purpose of the NEC mandate on combiner breakers is to provide overcurrent protection.

--vtMaps

Overcurrent protection for reverse current where one failed string sinks current from two or more working strings.
But also used to isolate a string for testing or maintenance, which means interrupting forward current.
Looks like two different directions to me.
There will never be overcurrent in the forward direction from a panel or string of panels unless a power line falls on them, so specifying a breaker or fuse value with that in mind makes no sense.

I think that Midnite had to make a difficult choice and elected to have the breaker survive multiple switch uses without degradation as a tradeoff for perhaps destroying itself when opening once under reverse fault current.
The only completely satisfactory solution is to use non-polarized breakers and switches.

Vic

Re: Panel voltage and best voltage for charging (2 8D) 12V batteries

Some of the better (IMO) CCs -- MidNite Classic and the OB MX-60 -- allow the CC to be switched to OFF electronically, or in the case of the MX, commanded to Restart. This allows the input breaker to be switched with no current flowing.

Then there are the MidNite Panel Mount breakers (MNEDC), which appear to be non-polarized, for those who worry.

However, it has been my opinion, that interrupting non-fault "reverse" currents with the Polarity-sensitive breakers, like the MNEPVs would be very much less traumatic on them, and generally non-damaging to the breaker after many, many cycles. Seems to me that the level of trauma would be scaled with the magnitude of the current. But this is just my guess.

inetdog, have missed seeing your posts for many months, until quite recently, nice to see you posting again. Opinions, Vic