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I need a 36V battery for a project of mine. Eventually I will buy a li-ion pack but now when prototyping and testing I don't want to spend a lot of money so I'm looking at cheap alternatives.

I'm considering making a 36V battery from 3 12V lead acid batteries. I found 9Ah ones that would cost me about $30 total but I don't also want to spend that much on a charger. I have a car battery charger for batteries up to 65Ah (6A max). I'm thinking if I put my 9Ah batteries in parallel that would draw around 3A I suppose which should be fine for the charger but I'm afraid if it's going to be able to maintain the proper CVCC modes because it wouldn't know it's charging 3 batteries and it most certainly wasn't designed for this.

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  • \$\begingroup\$ Should not be a problem to put them in parallel, unless their initial voltage difference is too large, and charge them that way, but you are aware that you will need to break the series connection before you can parallel them for charging? \$\endgroup\$
    – winny
    Jun 8, 2020 at 9:37
  • \$\begingroup\$ @winny yes yes, that's kind of inconvenient but for this ridiculously low price I don't mind. \$\endgroup\$
    – php_nub_qq
    Jun 8, 2020 at 9:38
  • \$\begingroup\$ Then no problem. Just make sure the voltage difference isn't too large before connecting them in parallel. \$\endgroup\$
    – winny
    Jun 8, 2020 at 9:39

1 Answer 1

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Charging Lead-Acid batteries in parallel is not a problem, but a larger battery charger that feeds up to 6A, when equally divided by your 3 batteries of 9Ah, would give 2A/battery, in the best case.

Sealed-LA batteries maximum charging current usually is 10-20% of rated capacity. For a 9Ah, I_max = 1A ~ 2A. In the present case the above 2A (or more) is a tad high.

A simple suggestion: use a current limiting device as a resistor, or just a 21W break-light 12V car lamp - one per battery. In the case of the 21W lamp, the worst case current will be 1.5A, but more probably about 1A per battery circuit, controlled by the lamp non-linearities. To better estimate the current, just measure it.

When using these resistors/lamps, each charging circuit will be safely limited; even if batteries are not equalized and have voltage differences, this will not be a problem and the same final voltage will be assured per your charger control.

Update 18/Jan/2022: “php_nub_qq” answered he proceeded charging them in parallel, where charging current was seen as “1~1.5A (total)”. In this case, limiting resistor/lamp might not be needed. On the other hand, in case one battery is “weaker” than other two, it’s internal resistance may be higher, receiving less current. For the same charging time, it is equivalent to be charged with lower accumulated Ah, becoming weaker, charging less, a vicious cycle.

To somehow minimize the undercharging, keep charging at Floating voltage for a longer time - assuming the charger operates with this Floating feature - further information here. I would suggest Float charging for at least 20% more time than (Batt.Capacity / Chg.Current). For example: 1,2 x (3x 9Ah / 1.0A) = 32h >>> “3 full Days”. If the charger is Floating type (with V.ch = 13.6~14.0V) it will not damage the battery even if left charging for few extra days.

It is recommended to do a follow-up of each battery charging current and rest voltages - before & after charging, if you wish to better understand batteries’ aging. We all would appreciate to know and learn from your findings - maybe you could do an update with them.

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  • \$\begingroup\$ Been some time since I posted this question. I actually went ahead and did it and it appeared to work. There is an integrated amp meter in the charger which showed that all 3 batteries were drawing about 1~1.5A (total) which is way less than expected. In that case do I still need to be worried about current limiting? \$\endgroup\$
    – php_nub_qq
    Jan 17, 2022 at 11:52
  • \$\begingroup\$ Probably not to be worried (at the moment). Based on the values presented, probably your Depth of Discharge (DoD) is not too intense. I would suggest you measure battery voltage - same as in parallel, but might work as separate items - not 100% remembered here. And plot/table the Current x Voltages. This will give a hint of internal resistance (VxI; charging/rest/discharge) and its service life. \$\endgroup\$
    – EJE
    Jan 18, 2022 at 14:13
  • \$\begingroup\$ Ok, I read it again: Batteries are used in series to make 36V, but charged in parallel, with a charger up to 6A total. DoD will depend of actual AH capacity of each battery, as draining current is the same (36V series). If one ages different than other, lower Ah capacity and lower voltage can happen. Then my suggestion to balancing may extend the life of the weaker one. \$\endgroup\$
    – EJE
    Jan 18, 2022 at 14:24
  • \$\begingroup\$ Also as a “weaker” battery has higher internal resistance, charging current may be lower, so the same time of charging time should be longer to have the same Ampere_x_Hour sent to that weaker battery. \$\endgroup\$
    – EJE
    Jan 18, 2022 at 14:35
  • \$\begingroup\$ You said the Total current is 1.0~1.5A. But what about the voltage for this charging value? And which were the voltages Before and After being charged? Preferably measure them at Battery terminals. \$\endgroup\$
    – EJE
    Jan 18, 2022 at 14:39

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