Battery backup 12v 5a

Hi everyone,

Here's another battery backup question! Consider yourself warned!

I've been trying to read up on battery power backup for a couple of weeks now, and I found information on battery charging modules (like 18650) that connect to li-ion batteries which are then connected to the load to supply power in case of main power failure.

The issue for me is that the batteries and charging modules I came across don't handle as many amps as I would like.

I am using a couple of car lock actuators that use 12V and about 5 amps each, in addition to the amp draw of other components (really not as much of a factor when compared to the 5 amp needed by the actuator).

So here's what I'm hoping someone could help me with:

What is the best type of battery to use? And has anyone used a good battery charging module that can handle 5 or so amps? Power failure is not common here so the battery backup solution will not be used much, and when required, will only be needed for a short period of time.
The battery itself isn't too much of an issue, I can get a 12v lead acid battery with 5ah, but I need a qualified charging module for the job. Any recommendations?

Also, I would need a couple of diodes to ensure the proper flow of electricity - what would be the best diodes to use? I know I have to look at max voltage flow (+ buffer) and max amp, so would something around 18v and 6amps work? Do I need to allow for higher amps?

Your help would be greatly appreciated!.

Thank you

Simple/safe solution when combined with a motorcycle maintenance (trickle) charger.
You probably don't need anything else if you power Arduino/lock from that all the time.


I use this family… they’re great, and there are several smaller*larger in the series

12V SLA charger

lastchancename gave you a good starting point. I would connect the motors directly to the batteries, they are designed to operate while the vehicle battery is being charged. Since this is an arduino forem I would simply using a Arduino UNO powering it via Vin from the battery and use low side switching for driver such as MOSFETS with a UIS rating and at least 30 amps. These motors can draw a lot of current when starting or stalled. If you have to reverse it consider a BTS7960 Bridge they are not that expensive, less then $10.00 delivered.

Thank you all for thr replies.

Lastchancename and gilshultz, when charging the battery, does the module protect from overcharge/discharge? So how should that be wired? Mains and battery connected to module, and you are suggesting to connect the actuator to the battery? Is there a separate Load line going out of it? I found a 5amp version but it's not cheap!!!

Just to give you more info, I have the door lock actuator like this

And that is connected to a motor driver

So I would have the power(main/battery) feeding that motor controller. I would also have the power going in to a step down converter then to an esp32.

Thank you for thr suggestion Leo! interesting, I didn't think about about a motorcycle trickle charger.

So you're suggesting I connect it this way:

mains -> trickle charger -> battery
Then branch off to
-> motor driver -> lock
-> step down converter-> microcontroller

Can I go from main to main load and mains to trickle charger then battery then load with a couple of diodes I don't want to go through the battery if I don't have to

Thanks a lot

Can these be replaced with lower current motors ?

I initially considered other options especially servo motors. Not sure if I want to switch to other options at this point. These actuators are more sturdy and seem a lot more reliable than thr little servos I was playing with... To be honest I didn't put too much thought into the battery backup back then. It would definitely be cheaper and I would have more options for components (smaller size too) if I had selected a less demanding motor.

If I can't find an affordable and decently sized solution, then the other option I have is to Not power the actuators, but to use a smaller battery to power the esp32 and have it notify me of power failure. What is he best way to detect that power is being supplied from the battery - that is, there is main power failure -> trigger an alert?


The driver you link to will not handle the current of the lock motor. You do not need a step down converter for the Microcontroller (UNO) it already has a regulator on board and will work from 12V.

I'm actually using an esp32 not uno. I'm not sure if I sent he right link for the motor driver but I'm pretty sure I'm using that exact one (I'll double check later), but either way it works well. I just need it to activate for a very short amount of time , just long enough to either open or close the lock

I suggest you pick an appropriate driver, your choice is not strong enough. You show the L298 stepper module, It israted its max as follows:
Peak Output Current (each Channel) 3 Amps
– Non Repetitive (t = 100ms) 2.5 Amps
–Repetitive (80% on –20% off; ton = 10ms)
–DC Operation 2 Amps
If you plan on using it buy several it will not survive reliable. Your motor is rated at 5A, how much will it pull when starting to lock/unlock?

It will just pull/push a little metal piece for locking/unlocking purposes, so the weight it needs to deal with is not much at all.

I agree, it might not be ideal for 5 amps but it seems to work fine (for now) for my application, it really needs to activate for less than a second (I forgot the exact length of time I put in the code). I think I could have used a servo but using this actuator seemed straight forward (pardon the pun) and it activates quicker.

For the battery backup, I still have a couple of outstanding questions

  1. Battery: i think I'm good here. I will probably go with 12v SLA, 2ah. I only need it for emergency situations and not for any extended period of time

  2. Charger: What charger specifications should I pick? I think that might depend on point 3 below.

  3. Circuit: if I connect things as @Wawa
    suggested then it would be AC -> charger -> battery and battery-> load but if that's the case then the charger should be able to supply at least 5a, but that might be too much for the battery. What charger should I use?

The module that @lastchancename Recommended would be perfect but is too pricy for me since I might never actually use backup power.

How about

Main power and split

  1. To load using an adapter - AC to 12V DC 5a
  2. To charger and battery which then connects to main load with a diode.

For this I would need to secure the AC power and use a terminal block to distribute the power to the adapter and to charger

Can I even use capacitors, just enough to get the microcontroller to notify me? And have it maybe store enough power for one time activation?

Any new info would be appreciated.

If you need backup power, that argument is irrelevant…
Using the same terms, you probably won’t need backup power, so why bother.

The cheapest option is a ‘big enough’ primary pack, and a diode, no charger. If it’s never used, replace the cells every couple of years…

The charger size doesn’t really need to be any bigger than the battery if you size everything properly… the system is normally running of the battery, the charger simply keeps it topped up

1 Like

Don't confuse Ah with current draw.
A 40Ah car battery has no problems supplying 250Amp starter motor current.

There are special plug-packs for charging lead-acid batteries.
500mA should be more than enough for a 2Ah battery.
2Ah seems small though. 5-7Ah are the more common types.

For a few seconds the battery will supply the current, it will give you many cycles. Look at the battery specification. See what its discharge capacity is. Remember you only need it supply for a short time. However that energy can destroy your H-Bridge when the motor turns on.

@lastchancename and @Wawa , regarding the size of the charger, what I meant is that there should be a max safe charge going in to a 2ah battery, more than 5 amp charger (for example) might not be a good idea. Now if I connect the charger to battery to load and my load is 5 amps, but my charger output is 1-2 amps when charger is connected to battery which is connected to load at the same time, where is electricity being really sourced from? Would it be the battery itself? Or the charger? The battery has 1 - and 1+, so both the charger and the load would be connected to them. What would be feeding the load? Would the load be taking electricity from the charger? The battery? Or acombination of the two?

I understand, that's why I'm thinking a 2ah battery is more than good enough for my 5(ish) amp load.

My primary objective to all this is to be notified if I ever lose power. So at least, I need one last breath form the microcontroller to let me know power is lost. My secondary objective, which is nice ro have, is to have a bit of backup power in case main power is lost for a longer period of time (I haven't determined what that acceptable period of time is yet). Since backup power is mostly nice to have, I want to just minimize the cost.

Your suggestion for a non-charged battery with a diode might be the best compromise, but before I take that route I will still look in to the charger option, even if it's just to learn more about how all this fits together.

Thanks again for thr replies!

I don't remember why I decided to take this risk to be honest. Maybe I was hoping the very quick activation can go through before the motor driver gets mad at me. For now it tolerates me, but I don't know for how long. Maybe I should look for something more suitable. Something like this should do the trick

Any other good ones that you recommend?

Thank you!

Ok, I realise those charger modules aren’t cheap, but neither are SLA batteries…
I have quite a few of those chargers with 12V 4.5Ah batteries in 24/7 operation in agricultural applications.

The projects have multiple sensors, 12 & 5 V peripherals, relays and a GPRS modem on board.

They just keep running, many with >12 month uptimes. (I’m not sure why they’re ever restarted ?!)

Look at the BTS7960 modules, they are less then $10.00 to your door from your favorite china supplier. These will more then supply the current the motors need. It could drive both of them concurrently if you want.

1 Like