A few years ago we could see a lot of solar battery bank for week autonomy in domestic installation. Today the price of raw material and the explosion of the demand multiply prices making the total autonomy a luxury impossible to get.
Yet solar batteries are far the frailest component and a mistake in the size or the plugging can quickly get fatal. So you have to find the best compromise between the maintenance level, life time (in cycle) and the price of batteries that you choose.
The first criterion is the maintenance level that can be provided on the site. It is primary to have someone for the maintenance and who can watch over the all system and batteries in particular. This person must have a minimum knowledge or being trained during the installation.
As installer we saw a bunch of mistakes, little or fatal… such as putting mineral water (death of the battery) use solar battery for something else like cars (death + or – slow), change schema of installation, don’t fill electrolyte level (fast death)…
So if you are not present on the site and check the maintenance by yourself or don’t have somebody to do it for you, we recommend you to use solar batteries without maintenance and don’t need distilled water.
For little application, like simple lightening, we can use car batteries; they are shorter in life time but a lot cheaper than solar batteries, so hard to make difference at the end. But still, take good quality battery.
For capacity that goes from 100Ah-12Volts to 800Ah-48Volts, it’s possible to work with 12Volts batteries. The rule is do not get over 4 batteries plugged in parallel.
So for bank with more than 800AH-48Volts we would have to take batteries with 6 or 2Volts elements, industrial type. Plus it is the best life time batteries, but they are big, heavy and more expensive.
However if you want good installation with competent people for maintenance, those batteries have a long life time because of their many cycles.
Size of batteries comes after size of the consumption.
Click here to open page about size of battery consumption
So once you have made your consumption statement you have to choose which autonomy you want. In Madagascar and Africa in general, rate of period of sunshine is such that there is no need to have many days of autonomy, generally one or two is enough. Depending of the energy you want to stock, you can choose the better batteries for your needs.
Ex: day consumption = 1200Wh x 2days of autonomy = 2400Wh that you need to stock.
Batteries stock Amps, it has to be transform into Watts by dividing the power (Watts) by the voltage (Volts);
P = U x I so I = P / U
So we find 2400 / 12 = 200Ah
With this number you have to add a lost coefficient linked to the electrical chemical transformation, between 30 and 20 %, depending of the battery quality.
200Ah + 20% = 240Ah
To this new number we have to add a rate of maximum unload, because it is impossible to fully unload a battery. Generally a value between 30 and 40% of energy don’t damage the life time of the battery.
X x 30 / 100 = 240 so 240 / 30 x 100 = 800 Ah
To stock 2400Wh we need 800Ah with 12Volts, which we can get by putting four 200Ah batteries in parallel.
For those who don’t remember the difference between serial and parallel, here is a little reminder;
Putting in parallel is to add amps by plugging + with + and – with – .2 batteries of 100Ah 12Volts in parallel give 200Ah in 12Volts.
Putting in serial is to add voltage between them, 2 batteries of 100Ah-12Volt gives 100Ah-24Volts
Increase voltage by putting batteries in serial gives the advantage to decrease amperage and so size of cable and components
But a 12Volts system is simple and we can directly connect 12Volts applications (lighting, radio, tv, solar fridge ...).
In conclusion, everything depends of the size of the installation and the componentsused.
You have to know that batteries are the weak part in a renewable energy system. They are more and more expensive and are still really fragile.
Bad settingand they can be quickly under charged or over charged, they can overheat, oxidized, well a lot of things can happen…
We have noticed that batteries could last longer if the loading had been well adjusted and if a better quality selected.
First step is to choose between flooded batteries or wet batteries.
Flooded solar batteries
There are many reason that can cause the early end of a battery.
Overload, under load, extreme heat, not fully download or not often enough, mistake with water…
Principal cause of battery premature death is the forgetting of demineralized water add, so it’s better to prefer wet battery, also called “without maintenance” or “VRLA” for Valve Regulated Lead Acid.
But to be without maintenance ask a little bit more care.In opposition with wet batteries, flooded batteries can get electrolyte made with gel (GEL batteries)or electrolyte that works on closed circuit with cyclic gas recombination made during cycles (AGM for Absorbant Glass Matt)
-Because we can add water in cells, we just have avoid two things:
-first of all, do not warm them too much becauseto water can evaporate by regulation valves that work security valves.
-In second place raising too much the voltage which produces too much important gassing with for consequence a dryness of electrolyte mixture and a quick death.
ADVANTAGES of flooded batteries:
Less important unload than wet batteries, no maintenance, no need of equalizing load, many cycles (depend on the maker)
INCONVENIENTS of flooded batteries:
More expensive, at least 35 to 50%, frailest and don’t get big Amps, don’t like big heat (can’t exceed 35°C), not made to have big load-unload cycles and prefer to stay in a small voltage bracket.
Wet solar batteries
However wet batteries, even if they need more maintenance, they are more resistant:
More resistant of the heat so can handle bigger Amps, less delicate, faster with the loading, can have higher voltage, handle bigger unload, cheaper.
Cannot be used in windless places, big auto unload, need to be loaded more often, need equalization load so need more complex equipment.
In conclusion we shall prefer flooded batteries if we do not want to bother with maintenance or if we are in isolated site with difficulty for access. It’s better for boat because they won’t lose acidity.
For installation with a lot of voltage movement such as in domestically installation; batteries are fully loaded during days and unloaded on maximum during night and that makes a complete cycle.
In this case, we would choose lead wet batteries, but need a rigorous maintenance.