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I have two VRLA batteries, 2Ah and 7.2Ah in capacity. Both are 12v. What would happen if I connect them in parallel when they are charged to the same voltage? can I use them to power a 10w LED? what will be the effect on the two batteries in parallel when drawing current from them? (These are to be used with a 10W solar panel)

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    \$\begingroup\$ Everyone is going to say no. Don't do it. And you probably shouldn't. But it has a good chance of working OK, at least for a while. I think something bad might happen when the batteries start to go bad. You could end up in a situation where all the charge current goes into the small battery and causes it to overheat. You wouldn't know unless you were monitoring temperature. So I advise you not to do it, too. \$\endgroup\$ – mkeith Nov 23 '16 at 4:00
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    \$\begingroup\$ Since batteries with different size/type/capacity have different internal resistance, there will be an imbalance in terms of discharge and performance when they are connected in parallel. This will result in discharging one of them faster and make it even overheat. \$\endgroup\$ – Rohat Kılıç Nov 23 '16 at 5:27
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    \$\begingroup\$ Will work just fine. Just be careful when initially making the first connection between the batteries. Miniscule differences in SoC (voltage) can cause excessive current to flow from one battery to the other until they balance out. You may want to temporarily put a load between the batteries to limit that current. After a while, the batteries will have the same voltage and will charge and discharge in parallel just fine. \$\endgroup\$ – JimmyB May 15 '17 at 12:45
  • \$\begingroup\$ Huh? What's a "VRLA" battery? I'm guessing LA stands for Lead Acid, but that still leaves VR. This is a international list, so abbreviations should not be used unless they are very common or universal. "Ah" is OK, for example. \$\endgroup\$ – Olin Lathrop Jun 19 '17 at 10:51
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    \$\begingroup\$ "Valve regulated lead acid" \$\endgroup\$ – IshanFdo1 Jun 19 '17 at 11:03
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i'm not an expert but...

"Most battery chemistries lend themselves to series and parallel connection. It is important to use the same battery type with equal voltage and capacity (Ah) and never to mix different makes and sizes. A weaker cell would cause an imbalance. This is especially critical in a series configuration because a battery is only as strong as the weakest link in the chain...

A weak cell may not fail immediately but will get exhausted more quickly than the strong ones when on a load. On charge, the low cell fills up before the strong ones because there is less to fill and it remains in over-charge longer than the others. On discharge, the weak cell empties first and gets hammered by the stronger"

you need only 1ah to run a 10watt load on a 12v battery. your 2ah will last 2 hours from a full charge if you use only that. if you need it to run longer then the 7.2AH would last most of the night.

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    \$\begingroup\$ It's just fine to put different batteries (capacity) in parallel providing they are the same technology (all lead acid all LiPo all NiCad etc), You don't need balancing electronics and cannot overcharge a smaller capacity one in parallel with a larger capacity one. Because they are connected together the terminal voltages track intimately and the batteries self balance. When being discharged the same intimate terminal matching ensure you cannot disproportionally discharge one battery. Many of the laptop batteries you see contain both series and parallel strings of cells. \$\endgroup\$ – Jack Creasey Nov 23 '16 at 4:19
  • \$\begingroup\$ @JackCreasey, and I heard just opposite. Each battery cell has slightly different parameters due to manufacturing variabilities, even in the same batch, due assembly variations, etc. The only reason the parallel sections in laptops work (and this is a questionable assertion, there are zillions of partially dead laptop batteries around) is because battery manufacturers carefully characterize and bin production batches, and only well-matched cells (impedance, charge-discharge profile, terminal voltage) make it into parallel sections. \$\endgroup\$ – Ale..chenski Nov 23 '16 at 5:11
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    \$\begingroup\$ @Ali Chen. Providing the batteries are the same type (chemistry) their charge and discharge characteristics will be similar enough to have no problems. You can read more here: batteryuniversity.com/learn/article/… ......the only real concern occurs in LiPo batteries where a cell that shorts would become a heat sink for current from adjacent cells. Even then it is very customary to see serial string connected in parallel. The RC environment (who use 3+ C discharge rates have been doing this for years for both NiCad and LiPo. \$\endgroup\$ – Jack Creasey Nov 23 '16 at 5:22
  • \$\begingroup\$ @JackCreasey, could you put some quantification on what is "similar enough" and what constitutes "no problem". If two battery cells (in parallel) have different shape of discharge function, and, say, left in a laptop at 60% charged, will it constitute somewhat faster self-discharge than carefully matched cells? The layman article does not address this concern. \$\endgroup\$ – Ale..chenski Nov 23 '16 at 6:43
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    \$\begingroup\$ But depth of discharge affects in battery lifetime a lot. So connecting them on parallel will allow me to reduce the depth of discharge on each battery \$\endgroup\$ – IshanFdo1 Nov 23 '16 at 12:53
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Be careful when you say charging batteries to the same voltage, as you can see in the picture below, a lot of batteries have a pretty consistent voltage output even though the state of charge is definitely decreasing, because of this voltage is usually a really bad way to compare charge.

Better way is to charge and deplete batteries, measure the actual energy going out and determining it in such a way. Also the changes in batch, build and all will definitely cause one of the batteries to charge/discharge more quickly then the other even if they were identical battery models.

From your question I understand you want to use the batteries as energy buffer/storage for your solar panels and power a LED of those, The easiest solution would be to just use 7.2Ah battery by itself.

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    \$\begingroup\$ When they are in parallel that's not a problem, right? \$\endgroup\$ – IshanFdo1 Jun 19 '17 at 11:01
  • \$\begingroup\$ If the two VRLA batteries are completely identical apart from the Ah rating, it will be perfectly fine. As long as the relationship between voltage and charge state is the same for both batteries it works. I just prefer safe then sorry, and with the age, deterioration and everything unknown. I dont really see the added value of the few Ah eventhough it will most likely work. \$\endgroup\$ – Remco Vink Jun 19 '17 at 11:12

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