Odd Battery Questions

Re: Odd Battery Questions

lazza wrote: »

HI Forum

I have some questions about a battery set up as i'm about to add 6 kyocera panels and regulator to an existing system.

The existing system has 5 parallel strings of 2x200Ah IEB 12-200 12V gel batteries- YES, I know, it's a recipe for disaster, but what's done is done.

As long as you keep the maximum charging current to around 5% of the total bank AH capacity--You should not be causing problems.

1. The old installer has connected some of the positive wires to the positive terminal of the 1st battery string, but he has connected the positive wire of the PV array to the positive terminal of the 5th battery string! A similar pattern is followed with the negatives. I always thought all the positives should be connected to the same positive terminal (1st string), and all the negatives too (last string). But maybe it doesnt matter as they are all connected in parrellel anyway? what are the consequences of what he has done?

As long as the current paths are balanced, you can connect to multiple locations... However, most configurations that I can think of only have two different "balanced" location points--So random connections probably are not great.

If the current requirement is low--it may not matter too much. If you have high sustained current, then the unbalance becomes worse (i.e., you draw/charge more current to the closely connected batteries, and the others are not).

2. All the strings were giving the same voltage of 24.2V at the time of measurement. However, in the 3rd string the two battery voltages were split 11.1V and 13.1V (in all other strings the individual battery voltages were at 12.1V)- What are the reasons for the voltage splitting like this between the two batteries in this 3rd string??

First off, the 24.2 volts is probably around 50% discharged--So you should be looking at getting the resting voltage closer to 12.7 volts instead.

Second, the 11.1 volt battery may have a shorted cell or the 13.1 volt battery may have a high resistance cell (or one battery is dead and the other is overcharged). Probably one, if not both, are toast.

I always suggest getting an inexpensive DC current clamp meter for folks with paralleled batteries so they can monitor load/charging current to find problems before they damage the battery bank.

3. Final question, nothing to do with batteries actually. We are putting in 3 strings of 2x12V Kyocera 120W panels, hence the system is at 24V. The new panels come with one cable of 10mm² (#8 AWG), yet the cable size needed is only 4mm² (#12 AWG) -at least until the junction box. I know you are supposed to keep the cable sizes the same, but is there any harm in using the existing short 10mm2 cable for the connection between -ve of panel 1 and +ve of panel 2 of each string (to make the string), but using 4mm2 for the cable runs from the +ve and -ve of each string to the junction box????

Nope, not a problem. Since solar cells are really current sources (unlike batteries which are voltage sources), they do not usually have a problem with "current sharing" because of slight differences in voltage drop/wire resistance with paralleled connections (unlike batteries which can be killed by such problems).

Hope this helps.

-Bill

Re: Odd Battery Questions

Larry,

A battery tries to keep (for example) 12 volts on its terminals... The lower the resistance, the more current can flow. If you have two batteries in parallel and there is different resistance in the connection wiring, the battery with the lowest resistance wiring will supply/take the most current.

A solar panel on the other hand between ~Vmp and 0 volts on its terminal will simply output rated current (Imp). So if you have several panels in parallel, and the output voltage is Vmp or less, any resistance in the current path does not matter. The panel with the lowest resistance path will supply Imp and the panel with the highest resistance path will also supply Imp.

Of course there are limits. A 12 volt battery's output voltage will collapse if you load it with too much current. And a solar panel will supply less current if its output voltage is over Vmp--And supply zero current if its output voltage is Voc (voltage open circuit).

And, the output current of a solar panel is proportional to the amount of sun hitting it... 1/2 full sun (~500 Watts per sq. meter), it will output 1/2 rated Imp (still as a current source).

Technically, an ideal battery has zero output resistance and and ideal current source has infinite output resistance.

Kind of mind blowing that your solar panel has a very high output resistance--Isn't it. But it make sense mathematically. One way of determining resistance is to measure the change in voltage divided by the change in current (R=Delta V / Delta I). An example:

• Rbatt=(12.5 volts - 12.4 volts) / ( 6 amps - 1 amps) = 0.02 ohms
• Rpanel=(21.3 volts Vmp - 0 volts ) / (6.95 volt Vmp - 6.27 amp Isc) = 31 ohms (real 185 watt solar panel operating at 80% sun)

So, in the case of a smaller battery, a 0.1 ohm difference in wire resistance is very significant. For the solar panel a 1 ohm difference will affect sharing very little.

-Bill

Re: Odd Battery Questions

For a battery... Basically any load (or charging source) connected, the battery will remain around 12-14 volts (to its chemical and physical limits).

For a solar panel, full noon time sun, 1,000 watts per sq. meter, it will, between zero volts and Vmp, output about Imp worth of amps (with Imp being roughly proportional to the amount of sunlight hitting the panel. Shine 500 w/sm and it will output roughly 1/2*Imp, etc.).

If you connect a solar panel (correct polarity) to a battery. The Battery will set the "system voltage" (12 volts or whatever). And the Solar Panel will set the charging current (~Imp).

We are so used to using a volt meter to measure line voltage (12 vdc, 120 vac) to see that everything is OK--We really don't get the idea that a random solar panel powering a random load will not have a set voltage output but will have a set current output (as long as the output voltage is between zero and Vmp volts).

-Bill

Re: Odd Battery Questions

You can measure the voltage at each battery and see if you are getting too much spread in voltage (0.1 volt is probably too much spread--0.01 to 0.05 maximum is better).

It would be interesting to see the difference between the 8 amp and 2 amp batteries.

If the voltage is the same on each battery (for the parallel connection points)--then those low amperage batteries may be going dead.

Also, monitor the current spread over time--It is possible that they will balance out somewhat as the high amperage batteries become fully charged.

It could also get worse--if the high amperage batteries get significantly hotter than the low current. High battery temperature drops the charging voltage at the battery (causing it to possibly draw even more current).

At this point, swapping high current for low current battery positions is probably a good place to start. See if the current flow is position or battery dependent.

-Bill