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I have 4 12v 20 AH, 2 12v 7 AH, and 2 12v 8.5 AH SLA batteries. If I were to put them in series to get 24v and then parallel to get 55.5 AH, would it be safe and would it work? Also are VRLA and SLA batteries the same thing? As the 8.5 AH batteries say VRLA on them while the rest are say SLA. I plan on putting them in a CyberPower CP1500PFCLCD UPS.

Diagram

Okay then, what if I just used the SLA batteries and not the VRLA batteries? Or should I just stick with the 4 20AH batteries?

Okay, seems like this is a bad idea, ill just stick to the default batteries inside of it. Id rather have a house to come back home too than an extra 30-45m of being able to use my computer in a thunder storm. Thanks!

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    \$\begingroup\$ Ideally, your arrangement works. You need same capacity for the series, and same voltage for the parallel. Just be sure to monitor the voltage of each cell in the series from time to time, to ensure that every battery is always at about the same voltage. \$\endgroup\$ – Vladimir Cravero Aug 23 '16 at 8:50
  • \$\begingroup\$ Okay thanks! Should I take any other precautions? \$\endgroup\$ – ArcticWolf_11 Aug 23 '16 at 8:52
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    \$\begingroup\$ If you're still stupid enough to do this: a fuse in series with each battery. And even then I strongly advise against doing this. \$\endgroup\$ – Bimpelrekkie Aug 23 '16 at 8:53
  • \$\begingroup\$ Yeah well I thought they were all the same chemistry, if this is not the case please refer to the answer below. \$\endgroup\$ – Vladimir Cravero Aug 23 '16 at 8:54
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    \$\begingroup\$ Okay then, what if I just used the SLA batteries and not the VRLA batteries? Or should I just stick with the 4 20AH batteries? They were all bought at the same time \$\endgroup\$ – ArcticWolf_11 Aug 23 '16 at 8:58
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Combining batteries which are not the same type and/or model and/or chemistry is always a VERY bad idea.

Combining SLA and VRLA is a NO GO.

You only would want to do this with batteries of the same type, model, capacitance and age. Combining any other way is asking for trouble.

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  • \$\begingroup\$ Okay then, what if I just used the SLA batteries and not the VRLA batteries? Or should I just stick with the 4 20AH batteries? They were all bought at the same time \$\endgroup\$ – ArcticWolf_11 Aug 23 '16 at 9:00
  • \$\begingroup\$ Read my answer carefully, especially the last 2 lines. Same type, model and capacitance. When placing batteries in parallel always make sure they're the same voltage. One SLA at 12 V and another at 11 V will cause VERY LARGE CURRENTS to flow as one charges the other. First connect them with a resistor or a car lightbulb in between to limit the current. Then when both have the same voltage you can connect them safely. \$\endgroup\$ – Bimpelrekkie Aug 23 '16 at 9:11
  • \$\begingroup\$ No two batteries have the same type, model, capacitance (surely you mean to say capacity?) and age. Even if they started out the same, capacity will diverge significantly over a few months. \$\endgroup\$ – Navin Dec 16 '17 at 4:09
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From YUASA:-

What does "VRLA", "MF", "AGM", and "SLA" stand for?

"VRLA" stands for Valve Regulated Lead Acid, "MF" stands for Maintenance-Free, "AGM" stands for Absorbed Glass Mat, and "SLA" stands for Sealed Lead Acid. All 4 of these terms basically refer to the same type of battery.

A VRLA battery is an SLA that uses valves to control the recombination of Hydrogen and oxygen back into water, after it was electrolyzed during charging. This allows the charging voltage to be raised slightly compared to a 'conventional' SLA battery, whose voltage must be kept below the electrolysis point to prevent gassing.

Most (all?) modern SLA batteries are VRLA, so your 'SLA' batteries probably are VRLA. The two main types of VRLA are Gel (gelled electrolyte) and AGM (Absorbant Glass Mat). These have slightly different characteristics that make them better for use in certain applications.

A charger designed for 'SLA' batteries should work equally well with either Gel or AGM, but one optimized for AGM might not be the best for Gel. Your UPS uses 'SLA' batteries, but the type of SLA is not specified.

Can I connect batteries of different Amp-Hours in series?

No, but that is not what you are proposing (Your series banks all use two batteries which have the same capacity). Lead-Acid Batteries can safely be connected in parallel, provided they all have the same state of charge. So you should make sure that each of your parallel banks is fully charged before connecting them together. It doesn't matter if the parallel banks don't all have the same capacity, as they will share the load accordingly.

Batteries connected in series must be as close to identical as possible, to make sure that the individual cells in each battery charge and discharge together and are 'balanced' (have the same voltage). VRLA provides some balancing capability, by allowing higher voltage cells to gas while the lower voltage cells are still charging. Putting different capacity batteries is series will lead to disaster because the lower capacity battery will charge up faster and become grossly overcharged, causing it to vent and release gasses that cannot be replaced - and perhaps even explode!

Batteries lose performance and may go out of balance as they age. Therefore for best performance you should only combine batteries which have the same age and usage, preferably new batteries bought at the same time. Connecting random batteries together will probably give poor results, as the older weaker ones will not provide the expected capacity.

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Connecting batteries in series is only practical if the batteries are very similar. So if you know each of your pair of serial batteries (for instance the 2x 12V 55Ah) have the same capacity, you can do that. You might want to measure the available capacity of the batteries. You also must balance the loading process! Especially in solar / offgrid situations, where charging and discharging can switch often, any imbalance in the individual batteries will accumulate over time, leading to a serial pair where one is partially charged and one is discharged, leading to damage.

You will have major damage, up to melting copper and fire, if you put batteries of different voltage in parallel!

You will have permanently damaged batteries if you put batteries of unequal capacity in series!

Now connecting batteries in parallel is a different beast altogether. Up to a point, especially regarding power drawn, they will balance themselves. If little power is drawn, internal resistance plays a lesser role to cell voltage and things will balance out.

Now you can reduce this somewhat by putting half the batteris in parallel and put these two grids in series, better averaging out the capacity imbalance. If you measure the capacities in advance, you can select the battery for the 12V grids so as to optimize the balance.

All that said: If you are hell bent on setting this up, I recommend leaving out the 7.5 Ah and 8 Ah batteries. They will cause more problems than they are worth, contributing only 15% anyway. Then make two grids of 2x 20Ah and 1x 55Ah each, put these in serial after you charged each battery with a reliable charger separately.

During use, monitor the batteries to make sure your use case works with the parallel setup. Switch off charging and discharging periphery and check if all batteries at rest have the same voltage after some use.

If not, set up a schedule to separately charge all batteries from time to time, depending on the imbalance in voltage.

The small batteries do have the same chemistry, but different internal resistance and possibly a slightly different discharge curve. Therefore you can expect them to be more imbalanced than the rest, leading to a more defensive discharging strategy (lower power, less total Ah). Therefore they are not worth using in this context. If they were SLA too, one might think about it, but still, its only 15% more capacity for 40% more connections and failure points.

Also, you have to check the individual battery currents. There is no guarantee that the current is balanced in any way. At any given point, it could be drawn from only one battery, balancing out charge imbalances in the 12V grids. This means, maximum current drawn is the max current of any given battery. Another important reason not to use the small batteries.

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No, you can't connect batteries of different Ah in series with a good result. However you can connect batteries of different Ah in parallel using diodes.

As stated already you should only connect batteries of same type/age/brand in series. In parallel you should use diodes to connect the batteries to the UPS. The diodes prevents one battery from charging/discharging another battery.

And also you should add some sort of protection circuitry to stop discharging the batteries when the voltage gets to low. This is to avoid damaging the first battery cell that gets totally discharged.

You should charge each series separately. Or use diodes here as well. But that gets a bit more complicated getting the charging voltage correct. The diode voltage drop is different because different current running through the diodes.

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If you put diodes in between parallel batteries to prevent one from charging or discharging another then you are preventing them from maintaining a balance. Voltage wants to run from high to low until equal. The more the difference, the higher the current flow. So if one battery has a tendency to drain faster, its voltage will drop slowing flow from it to load, while the battery with higher voltage will have increased current going to load as well as lower voltage battery, increasing proportionately in an attempt to stay balanced. Different internal resistances might prevent this balance from happening with large loads so it would be important to use batteries with the same or near the same "c rating" and to make sure that you never exceed the sum of the 2 batteries c rating.

C rating determines the rate at which the current of a battery can safely be discharged. Example: 4c rated 5ah battery would allow 4 x 5 = 20amps continuous current.

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