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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5 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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tanis645 avatar image
tanis645 answered ·

A typical Energy Star Refrigerator that consumes less than 300kwh per year, may be able to run on a fully charged 12 volt 100 amp hour storage battery for 30 – 35 hours. This varies with the ambient temperature surrounding the refrigerator and the efficiency of the DC to AC power inverter that is being used.

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christianf250 avatar image christianf250 commented ·

for your information, my fridge take around 15% to 18% of my battery bank in one night, including one led lamp for around 5 hours. my bank is 24 V at 230 amp ( 4 battery of 6 volts, 230 amp).

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christern avatar image christern commented ·
And then you would have completely discharged the battery and as original question is about a Gel battery this would kill it within 5 cycles.
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kevgermany avatar image kevgermany ♦♦ commented ·
@ChristerN

@Christianf250

Please keep this friendly. Sizing batteries for fridges is not that straightforward.

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christern avatar image christern kevgermany ♦♦ commented ·
@kevgermany Thanks for notifying!

I had no intention of being unfriendly and do not wish to be so now either but when I was a newbie and read and trusted all things in forums I got so many answers that were wrong, dangerous and so on. I have since then, and now grown much in knowledge (even if I don’t know everything), been very skeptic to advices (not specifically in this thread) that are misleading. Hence my reply.

Back to the question.

DC Compressor-based fridges can require quite a lot of power at startup. Other technologies do not.

It is always difficult to know how much a fridge is consuming since it depends very much on the surrounding temperature and technology but there is one example above which is good.

(i made a comment on @tanis645 comment above, in a pretty harsh tone I admit, and if you do the maths that fridge would consume somewhere between 85 and 90 Ah during the mentioned 30 - 35 hours and if you would use a 100 Ah (GEL) battery that would be pretty much destroyed after a few cycles.)

To the question on connecting batteries in parallel. During my last 10 years where I have been helping “friends and family” with installations in boats and vans I have seen many good installations with more than three batteries in parallel. I believe 16 parallel batteries is the most. Yes, there are multiple things that you should consider before doing such big installations but I would say that some of these things to build an ideal installation are just not deadly important (i.e. identical cable length) when talking about lead acid batteries and could be replaced with other things (circulating batteries).

So, still waiting for the reason for not having more than 3 batteries in parallel.

Sorry for long post!

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JohnC avatar image JohnC ♦ christern commented ·
@ChristerN

If you want the answers to the 3x parallel issue, please ask a separate question rather than highjacking this one. In the meantime, perhaps hold back on recommending it to your mates.

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christern avatar image christern JohnC ♦ commented ·
Thanks for good advice, will do so, although it wasn't me who brought up the 3-battery limit and thought that errors shall not be stated in this type of thread without being met.
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kevgermany avatar image kevgermany ♦♦ christern commented ·
You have the reason in the document I referenced.
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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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