24 volt battery calculations

rrroae · January 2010

I think I may have messed up my calculations.

I just upgraded my old 12 volt system which consisted of the following.

- 8 220 amp/h 6 volt batteries run series parallel for total of 880 amp/h capacity at 12 volt
- 3 Kyocera 120 watt panels
- Solar Boost MPPT
- Trace 1512 inverter

My new system is as follows.

- 4 Crown 395amp/h 6 volt batteries run in series to give me 395 amp/h at 24 volt
- 4 Kyocera 205 watt panels wired at 24 volt
- Morningstar 60 amp MPPT
- Magnum hybrid 4024

Now my question is, in my original system I only had 360 watts of pv for my battery bank of 880 amps at 12 volt. I intended to upgrade my pv wattage so I wouldn't always be charging my batteries. From recommendations here, I was told 1 * 1 - 1 1/2 Battery amps to pv. So my intent was to go from 360 watts of pv to 820 watts and to go from 880 amps of battery storage to 790 amps. This would give me a good ratio of 820 watts pv to 790 amp battery.

Did I misunderstand that when you have a 24 volt system your amp capacity is doubled? Do my 4 Crown 6 volt 395 amp batteries give me 790 amps of storage because they're wired at 24 volt??

I'm guessing I'm going to have to buy 4 more batteries.

rrroae · January 2010

Re: 24 volt battery calculations

Another question.

Can I rewire my 4 Crown batteries to 12 volt even though my inverter and the rest of my system is 24 volt?

BB. · January 2010

Re: 24 volt battery calculations

rrroae wrote: »

My new system is as follows.

- 4 Crown 395amp/h 6 volt batteries run in series to give me 395 amp/h at 24 volt
- 4 Kyocera 205 watt panels wired at 24 volt
- Morningstar 60 amp MPPT
- Magnum hybrid 4024

Now my question is, in my original system I only had 360 watts of pv for my battery bank of 880 amps at 12 volt. I intended to upgrade my pv wattage so I wouldn't always be charging my batteries. From recommendations here, I was told 1 * 1 - 1 1/2 Battery amps to pv. So my intent was to go from 360 watts of pv to 820 watts and to go from 880 amps of battery storage to 790 amps. This would give me a good ratio of 820 watts pv to 790 amp battery.

I am a little lost... So back to basics. Normally, we recommend around 5%-13% or so of the C/20 Battery Amp*Hour rate from the solar panel.

Roughly, solar power wise (these are rough numbers--10-25% or so higher for PV panel deratings and controller losses would be fine):

29 volt battery charging * 5% * 395 Amp*Hours = 570 watts minimum
29 volt battery charging * 13% * 395 Amp*Hours = 1,490 watts maximum

So, your solar panels at 820 watts is fine--if a bit on the low side of the range.

Did I misunderstand that when you have a 24 volt system your amp capacity is doubled? Do my 4 Crown 6 volt 395 amp batteries give me 790 amps of storage because they're wired at 24 volt??

I'm guessing I'm going to have to buy 4 more batteries.

Amp*Hours is a confusing rating... It is ignoring the voltage. Remember the equation for power:

Energy Stored = Volts*Amps*Hours = Watt*Hours

So, if you have, for example 4x 6 volt batteries... When you put the 4 in parallel, the Amp*Hours goes up by 4x.

When you put the 4 batteries in series, the Voltage goes up by 4x.

In either case the Energy Stored is exactly the same.

Based on your current numbers, I would not add any more batteries at this time. You have a bit more than the minimum amount of solar panels needed for your current battery setup.

Can I rewire my 4 Crown batteries to 12 volt even though my inverter and the rest of my system is 24 volt?

Not quite sure what you are asking about here? The MPPT Charge controller can be reprogrammed to charge at 12 volts--but the 24 volt inverter would be useless in a 12 volt system.

If you want to take the batteries out and use them in a 12 volt application (say running a trolling motor) then wire them back to 24 volts to recharge them in your 24 volt setup--that is can be issue.

Your four batteries when in series, charge and discharge the same current each time (Amp*Hours). If you pull two out for running a trolling motor (discharge to 50%) and then put them back in your solar system to recharge--half the batteries will be fully charged and half will be 1/2 charged. To recharge the 2 half full batteries, you would have to way over charge the other 2 in the 24 volt string. Not good at all...

If you discharged the 4x batteries evenly as a 12 volt bank--it would not be as bad--but still not recommended. It is not always easy to ensure parallel battery strings discharge evenly--and you could back in a similar issue as in the previous paragraph (guessing that this was your question).

-Bill

rrroae · January 2010

Re: 24 volt battery calculations

Thanks again Bill. That was really confusing me.

Since you think I'm at the low side for PV, I think I'll add my old Kyocera panels in to give me 1180 watts of pv. You advised me on this before I bought my current system.

I checked my 120 watt Kyocera panels and they can handle the higher voltage.

BB. wrote:

However, the Vmp and Imp need to match (within roughly 10%) for your application when sharing series/parallel connections for optimum output power. So--we need to do some checking.

The Kyocera 120 watt panels are (remember, always double check the data posted by any anonymous person on the web--I cannot know for sure your model or that the website I ramdomly selected for a quick lookup is accurate):

Max voltage: 16.9 Volts
Max current: 7.10 Amps

The 205 watt Kyoceras are:

Maximum Power Voltage: 26.6 volts
Maximum Power Current: 7.71 amps
Open Circuit Voltage: 33.2 Voc
Short Circuit Current: 8.36 amps
Maximum System Voltage: 600 volts
Series Fuse Rating: 15 amps

2x 26.6 volts = 53.2 volts Vmp for two series connected 205 watt panels
3x 16.9 volts = 50.7 volts Vmp for three series connected 120 watt panels

The Vmp is not an exact match--but is within 10%--so should be OK to tie together (with appropriate series fusing) to a 2x205 + 3x120 series/parallel connection to a good quality MPPT controller.

rrroae · January 2010

Re: 24 volt battery calculations

One last question if anyone is available.

When we were wiring the Kyocera 205 panels, the only way we could do it with those connectors was to wire + to - to make a 96 volt array. Actually reading 131 volts inside at the combiner box(VOC = 33.2 per panel).

Is there any reason to be worried about such a high voltage?

BB. · January 2010

Re: 24 volt battery calculations

How many panels in series (and which panels)?

If Voc=131 volts when you are having cold sunny weather... Should not be a problem.

However, if it is Vmp=131 volts--then Voc may be over >150 VDC which would damage your charge controller (typical Vmax panel input voltage for a MPPT type controller).

MorningStar has a string sizing tool... Have you looked at it yet?

-Bill

rrroae · January 2010

Re: 24 volt battery calculations

4 in a series 1 string parallel using the Kyocera 205's.

According to that nifty little calculator, I could have 2 of these strings run in parallel.

Now I'm assuming I won't be able to add my 3 old Kyocera 120 watt panels since I moved up the voltage on the 4 Kyocera 205 panels, correct?

3 Kyocera 120 16.9 x 3 = 50.7 volts vmp
4 Kyocera 205 26.6 x 4 = 106.4 volts vmp

BB. · January 2010

Re: 24 volt battery calculations

I would guess that if you could get 3 more of the 120 watt panels, you could put 6x in series and they would work OK (Vmp=101.4 volts).

Otherwise, get a "cheap" 24 volt PWM controller to run 2x panels on your 24 volt battery, or a less expensive MPPT controller to run 3x of the 120 watt panels...

Questionable whether either solution is worth the money vs buying 4x 205 watts and going "whole hog" on the installation.

-Bill

rrroae · January 2010

Re: 24 volt battery calculations

BB. wrote: »

Questionable whether either solution is worth the money vs buying 4x 205 watts and going "whole hog" on the installation.

-Bill

For the price I'm getting those Kyocera 205's ($570 from our host), that's a pretty tempting suggestion.

Thanks again Bill for all the help. I really struggle figuring this stuff out and I can't tell you how much your help keeps me sane.

rrroae · January 2010

Re: 24 volt battery calculations

Today was the first day charging with the new panels ans something doesn't seem right.

Our Magnum system monitor was showing we were only collecting 17.8 amp at 12 noon with no load(inverter shut off) and full sun.

I checked all the connections on the panels and the settings on my system and everything looked right. Re-read all the manuals and can't see why we're so low.

Any ideas??

BB. · January 2010

Re: 24 volt battery calculations

I assume that was battery amps.

What was the solar panel volts/amps. State of charge/voltage of the battery bank, what was the state of the charge controller (bulk, absorb, etc.). For example, if the bank is at 14.4-14.7 volts, then that is the current that the battery will accept at that voltage.

Is the voltage at the bank the same as "read" by the controller (i.e., no voltage drop in charging cable).

-Bill

rrroae · January 2010

Re: 24 volt battery calculations

Yes, battery amps.

The panels are at 131 volts inside at the combiner box with no load. Not sure the solar amps unless you're talking about the IMMP which is 7.71 amps(per panel).

Not sure yet on state of charge. This evening I ran the generator for 2 hrs on absorption before I hit float and now am equalizing them so I can get my 100% starting point.

When we were at a peak collection today of 17.8 amps, the volts of the batteries were around 28. I don't have my multimeter to check my battery voltage against the monitor voltage.

BB. · January 2010

Re: 24 volt battery calculations

I was asking what the controller said the solar input voltage/amp was (such as 100 volts 4 amps).

What stage the charge controller things the battery charging is in (voltages are approximate; maximum current at <28.4-29.4 volts; maximum absorb voltage which is declining current into the battery bank, or 26.5-27.6 volts for floating).

It sounds like your battery bank is fairly well charged. The charge controller will output maximum current if in the "Bulk" charging stage. Other stages the current will be less.

-Bill

PS: You can try turning on your DC/Inverter Loads to see if you can maximum current from the charge controller).

24 volt battery calculations

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