Small rechargable cells for security backup?

Thread Starter

jospanner

Joined Aug 10, 2020
3
Hi,
I'm working on a custom security system for an older vehicle. I have an off-the-shelf GPS tracker that would normally be connected to the car's battery. The unit has no battery backup. It normally takes a straight 12 volt supply. I'd like to have a battery backup for this thing in case something happens with the electrical supply (it's an old mechanically fuelled Diesel engine and can run without a battery).

My first thought was to use something like an A23 battery, but those are non-rechargeable. A small lithium pack with a boost/buck converter and charging module was another idea, but I'd be a bit worried about the safety of that sort of set up (and the compactness).

The GPS tracker doesn't use a lot of current, and I can't foresee it needing to run for a long time without its external source - so capacity isn't a big concern. The battery would, however, need to last a long time while being topped up by the car's own power system - 99.9% of the time it would have a power supply.

So:
Small, rechargeable, reliable battery. Can you think of any good options for this?
 

Alec_t

Joined Sep 17, 2013
11,640
The GPS tracker doesn't use a lot of current, and I can't foresee it needing to run for a long time without its external source
Then why not use a non-rechargeable battery? If connected via a diode, the discharge current would be negligible in normal use of the system.
 

dl324

Joined Mar 30, 2015
11,506
Welcome to AAC!
The battery would, however, need to last a long time while being topped up by the car's own power system - 99.9% of the time it would have a power supply.
Unless you use a lead acid battery chemistry, you would have to provide your own charging circuit. I don't use many SLA batteries, so can't give any suggestions. I have some 6V SLA batteries that are about the size of 3 or 4 AAA batteries.

If you're willing to add your own charging circuit, then other chemistries can be used. Anything other than SLA is going to require multiple batteries in series or a boost circuit. What is the voltage tolerance of the GPS unit? I had a mechanic who used to use a 9V transistor radio battery to maintain radio station presets when he had to disconnect the battery.
 

Thread Starter

jospanner

Joined Aug 10, 2020
3
My worry about a non-rechargeable battery would be the possibility of self discharge. I might be overthinking that, though?

Welcome to AAC!

Unless you use a lead acid battery chemistry, you would have to provide your own charging circuit...
Hi! :) Charging circuits are fine! I don't mind getting into that, I know that there are pre-done modules for managing various chemistries so I was thinking of just going that route for now. I'm mostly concerned about the chemistry of the battery for long term and safe use. As for the voltage on the GPS unit, I think 9V would be pushing it. The thing does have a GSM radio on board and I can't imagine that being too tolerant of low voltage?
 

dl324

Joined Mar 30, 2015
11,506
I'm mostly concerned about the chemistry of the battery for long term and safe use.
Most battery chemistries don't like being abused. Li-ion has gotten a bad rap, but fires and explosions are more likely the result of poor manufacturing.

You could go with one Li-ion battery and a boost circuit to get it to 12V. You could have the boost circuit off until power is lost if you include a capacitor to power the GPS while the power fail circuit turns on the regulator.
 

Thread Starter

jospanner

Joined Aug 10, 2020
3
Most battery chemistries don't like being abused....
So as long as it's a decent cell with a proper charging/cutoff circuit, it should be good for the long term? Set-and-forget, so to speak...
18650 cells are easily available, they seem like a possibility. I'll have a look around and see if I can find (reputable) smaller batteries for this thing. Thanks!
 

Irving

Joined Jan 30, 2016
1,009
If you're worried about safety, use LiFePO4 cells, they pose no risk at all. 4 cells give you 12.8v nominal, 14.4 open-circuit immediately post-charging, so are voltage compatible with 12v SLA but 3 - 4 times the energy density - they are a better fit than LiPo for automotive voltages. They need a more complex charger than SLA but its not too difficult, and will give you 10-15y life if not misused too badly. Self-discharge rate is measured in months (some of my packs lose less than 5% in a year!) - far better than SLA or LiPo.

Another viable option would be to use supercapacitors @ 2.7v, a stack of 5 gives you 13.5v fully charged. They behave very differently to batteries but can charge virtually instantly and are good candidates for battery backup solutions. They have zero safety and longevity issues for this requirement, but I'd need more info to size them for you.
 

crutschow

Joined Mar 14, 2008
25,398
If we knew how much current the GPS module took (I know it's "not a lot"), and how long you want the battery to power it, we could make better suggestions.
 
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