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I am currently working on a school project that have a stack of 4 (12V 20A) lead acid charged by adapter from wall outlet. With 4 individual batteries, if i put in series, i will have 12V, 24V, 36V, and 48V output. My questions is how can i control the specific individual battery that i want to use and not use the whole stack of battery to step down to a desirable voltage level.

For examples: 4 of the batteries now named #1, #2, #3, and #4

The load is only ask for 15V, so now i only need to use 2 from the stack of 4 batteries. In this case, i will use battery #1 and #2. Then, i step it down from 24V to 15V, while keep the other 2 batteries charging.

When i notice battery #1 and #2 draining out, i can switch my source to battery #3 and #4. Then, charge battery #1 and #2.

Thank you.

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    \$\begingroup\$ why do you think this is a good idea? \$\endgroup\$ – Jasen Mar 30 '18 at 1:28
  • \$\begingroup\$ Read about balanced chargers and balanced loads \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 30 '18 at 2:04
  • \$\begingroup\$ Welcome to EE.SE. Look up 'voltage regulators'. That might help. \$\endgroup\$ – Sparky256 Mar 30 '18 at 2:54
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Well, sure you can, but that's not the same as saying you should.

Let's take your example. You've been discharging 1 and 2 while charging 3 and 4. Let's say that 1 and 2 are approximately 50% discharged, and 3 and 4 are fully charged. You're about to swap the two pairs and start charging 1 and 2.

At this point, your power requirements change, and you need 48 volts.

So you hook all 4 batteries in series and start discharging. At some point you've discharged 3 and 4 to 50%. That means that 1 and 2 are at 0%. Any further discharge will reverse-bias 1 and 2 and cause irreparable harm.

In other words, unless you are extremely cautious, you run the risk of destroying one or more batteries. How cautious? Depends on how badly you want to avoid blowing up your batteries. You'd need to monitor the performance of all 4 batteries and assign some sort of margin requirement to the state of charge for all batteries.

Do you really need to worry about this? I suggest you look up (if you are not familiar with it already) Murphy's Law. Words to live by.

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