It's an 8Ah motorcycle battery which I intend to use to power a the pump that waters my plants twice a day. The project has been working for 3 years using NiMH battery pack which I recharge every few months. I'm switching to a higher flow pump, hence switching to a motorcycle battery.

I bought two modules with a relay that cut off the load if the voltage of the battery drops under a set voltage but these modules have a high quiescent current, so I'm looking for an alternative.

I'm using a microcontroller to keep track of time and actuate the pump through a mosfet. I considered having the microcontroller measure the voltage and do nothing if it's below a certain value, however, I'm concerned the microcontroller get stuck somehow and keep the pump going until the battery is depleted.

PS: the pump pulls approximately 2 amps under load, would you recommend placing a fuse in series? what type?

 

Slow blow fuse, because it's a pump, so it's a motor and has higher starting current.
Placing a fuse is always prudent in circuits that are on everyday and without close attention.
I think 3 A fuse is ok. But I'm rookie when it comes to standards. But I think a fault would be drastic in terms of current, so 3 A fuse would do as well as 2.5 A one.

 

Recommend low-voltage cutoff circuit for 12V lead-acid battery

hi FB,
Check this link, clip added.

A 12V lead-acid battery fully charges at 14.2V to 14.7V and discharges down to a safe minimum of 10.5V to 11.9V depending on the load and state of charge.

E

https://www.google.com/search?clien...+discharge+voltages+for+12v+lead+acid+battery

 

If you don't have the battery already, buy a deep-cycle type.
They will last longer in your application.

You might also consider a LiFePO4 type (example below).
They are somewhat more expensive, but have characteristics similar to a lead-acid battery, and generally can last for many more charge deep-discharge cycles (usually by a factor of 5 or more).

But they do need a charger that works properly to charge LiFePO4 batteries.
Some lead-acid chargers will work, but you have to be careful about the voltage they charge to (14.6V absolute maximum for LiFePO4).

 

I'm using a microcontroller to keep track of time and actuate the pump through a mosfet. I considered having the microcontroller measure the voltage and do nothing if it's below a certain value, however, I'm concerned the microcontroller get stuck somehow and keep the pump going until the battery is depleted.

I am in favour of using an MCU (microcontroller). The default power on state should be the MOSFET is off.
Make the MCU mission critical. Use the watchdog timer.
The MCU can measure battery voltage under load. Also measure the run time and estimate Ah consumed.

 

I don't know if you (TS) are willing to program STM32, but if yes, you can schedule a timer to change from stop mode to running. So it consumes about 15 µA in stop mode, and few mA when running. It depends how much you want to embed it, or how much you want to go fully analog.
ESP32's RTC is not as accurate as the STM32, but as your application is not critical on time tracking, it might be a viable solution.
The perfect solution would be to add a fuel gauge, like BQ27441, or others that are sold as modules (to easily connect).
But you can do it more simple, like measuring the voltage with the MCU's ADC.
It's a tradeoff of how much complexity you are willing to put and then debug vs. how much simple you want to keep it but it might have quirks that are unavoidable and/or undesired.

 

I had a second thought about it, before TS answered about going full analog on the voltage cut-off circuit.
You can have a voltage reference like TL431 and compare with an op amp with the battery voltage. Simple comparator. There are lots of circuits like that. You can have a current as low as 0.5 mA for the TL431, plus the op amp.
But it depends if you want to prototype a board with that and buy the parts.

 

To do a worthwhile check do it while the pump motor is running. No-load voltages are higher and less likely to reflect the actual state of charge.

 

It's an 8Ah motorcycle battery which I intend to use to power a the pump that waters my plants twice a day. The project has been working for 3 years using NiMH battery pack which I recharge every few months. I'm switching to a higher flow pump, hence switching to a motorcycle battery.

I don't understand the problem.
If it's been working fine using NIMH battery pack what difference will it make using the motorcycle battery if it's charged when needed.
How was the NIMH battery monitored to know when to recharge?

 

The 'problem" that is seen is adquately knowing when the "motorcycle" battery needs recharging. NIMH battery performance and support are quite different from typical LEAD/ACID batteries such as the motorcycle battery. THAT difference matters quite a bit. Both the charging profile and the discharge profile are quite different from each other.

 

The TS seems to have left the building.

 

Hello,

@FranciscoB , i see you where online on thusday.
Please respond to this thread.


Bertus

 

As noted, NiCads can be run down until they stop working. Recharge and you're good to go. That's part of why they're the easiest to use in solar lamps. (They're also very tolerant of an over-charging current.)

That type of service will ruin a lead-acid battery although a deep-cycle will do better, again as noted.

I'm wondering what the TS would like the shut-off circuit to do - just shut off as if the battery has gone dead? Light or flash an LED to alert the user that a recharge is needed?

Maybe the solution is running a cord and attaching a battery tender (like a charger but low current, not meant for charging a dead battery) to keep the battery topped up. No recharging, hands off.

 

If you tap on the person's icon and again when the window opens the additional info is available.

 

There are battery protection circuit devices available. This is not a new problem. the fuse mentioned in posts #1 and #2: Put the fuses in line as
close to the battery supply as is REASONABLE! And a 3 amp, or even a 5 amp fuse will protect against any short circuit problems. Unless the battery is close to fully discharged.