question

komsolar avatar image
komsolar asked

battery size

I have a MPPT 75/15 charge controller charging a 1 x 150AH gel battery from 2 x 100w 18v 5.5amp panels (in parralel). Connected to the battery is a Victron Blue solar 12/1200 inverter. I was wondering if I could add a second 150Ah gel battery to create a 12v 300Ah battery. I'm new to all this, so was wondering if the charger could handle charging double what it is doing now? Currently the load on the inverter is around 300W, but I want to add my fridge (260w) and need the system to run for around 4 hours during loadshedding.

battery charging
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kevgermany avatar image kevgermany ♦♦ commented ·
Couple of things to add to @Seb71 answer.

I hear rumours of loadshedding getting up to 12 hours soon.

If the existing battery is older/heavily used, don't add another, replace with a new larger pack.

I think you've probably done it already, but estimate fridge % run time on hot days and size the batteries to suit based on the power draw on the fridge label. Add some for inverter losses.. Max discharge 50%, how quickly solar will recharge and so on.

Sounds, like seb said, as if the existing installation is too small.


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4 Answers
seb71 avatar image
seb71 answered ·

Sounds like you also need to increase your PV array.

Or extend the length of the day.

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christern avatar image
christern answered ·

Without knowing the specs of the inverter I would say that it is always good to extend the battery capacity. Bigger battery banks accepts charging better and tend to have much longer life due to less degree of discharging. Just make sure you charge the batteries fully from time to time.

Keeping the same system voltage is of course necessary and that is what you intend to do. So, yes!

PS

Add a Smartshunt or a BMV 712 to make sure you don’t discharge your batteries too much. Important even if you stick to one battery!

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christianf250 avatar image
christianf250 answered ·

Hi.
For your fridge do not forget the amp on startup are realy heavy for short period of time. with my small fridge 3.2 ft cu. when is working on 120 V ac the amp is arount 0.82A, but when it start the amp increase up a short period of time from 7.2 and 8.1 amp. (x 10 factor)
Your solar inverter are made for 12V system, if you want add battery you will need to place them in paralel, the limitation in parallel is 3 battery and you will need to interconect them carefully.
A fridge not running always it stop and start.

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christern avatar image christern commented ·

”the limitation in parallel is 3 battery”

What do you mean by this? There are no such rules!

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christianf250 avatar image christianf250 christern commented ·

in victron book is recommended do not use more than 3 battery in parallel. what I understand is the resistance of the battery are verry low, when you put many battery in parallel is ohms law apply : 1/r + 1/r + 1/r ... = 1/(answer) at the end the entire resistance are verry verry low, each connection, nut, and lenight of each wire affect the result. if the internal resistance of the battery are not the same of the other one, some battery will be drain more than other one and you will need to replace them, is few $$.

If you want put more battery in parallel than 3 is your choice.


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christern avatar image christern christianf250 commented ·

Thanks for explanation. Haven’t seen that in their documentation and nowhere else either so I disagree and could argue against all the reasons.

So, until someone has proven this I still don’t believe it.

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kevgermany avatar image kevgermany ♦♦ christern commented ·
Take a look at Victron's wiring unlimited. It's downloadable via Google. Section 3.2 covers and explains it.
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christern avatar image christern kevgermany ♦♦ commented ·

Thanks again @kevgermany for the reference. Haven’t seen that before.

I am a very stubborn old man (!) and disagree partly with what is written in that section of the wiring unlimited document. Document is, according to my experience, a bit too theoretical in parts.

Reason: All lead-acid batteries vary slightly in internal resistance and charging capacity. The good thing with lead-acid batteries is that the charging level equals out during float and they become all on a reasonably same level after some time in float. This has been proven I different tests. This goes, naturally, for batteries with same age, capacity, chemistry and brand and when correct cabling is used.

So, the “limit” of 3-4 paralleled strings I would say is mainly theoretical.

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komsolar avatar image
komsolar answered ·

Just a quick update, I spoke to an installer yesterday, he explained that with the panels I currently have, I should not add another battery to the existing one or put in a bigger one. I.E increase the Ah at this stage, as it would just increase the amount of time needed to charge it. The 200W panels are pretty close to the maximum my 75/15 charger can handle and advised I leave it as it is or increase the number of panels and get a bigger solar charger. Its working well as it currently is and so far the fridge is coping with the 2,5 hour power outages. However if the outages do increase drastically in time, then I will add more panels, charger and battery. Thanks for all the help.

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