12V Trickle Charger & Battery Backup power supply for GPS Tracker

[DerryDigger]

Hi, I've just got a new car and I wont be driving it too much, so I want to attach a trickle charger, also I want to install a tracker (12v) and a small lead acid backup battery.

Not to sure on how to wire this up, so I thought I'd ask people smarter than me.

I need to keep both batteries charged keeping the charger plugged in all the time the vehicle is not being used (existing car battery & small 4.5amphr backup battery, with portable trickle charger which is a NOCO Genius 2 Battery Charger 6V/12V 2 Amp),

keep the tracker with power all the time and make sure that if main battery is disconnected the rest of the car wont take power from the backup battery, (i only want the backup battery to power the tracker only).

Is anyone able to help me out with this, I've sort of got an idea how to do it (see attached file), but worried that I might over charge the backup battery when charger plugged in

Thanks
Derry

 

[wwfeldman]

Hi, I've just got a new car and I wont be driving it too much,

what is too much?
or what is not enough to keep the battery charged so you don't need the backup battery?

presumably, the tracker only needs to operate when the car is operating

i expect that driving the car once a month for about 20 minutes on a highway will keep the battery sufficiently charged that you will be able to start the car. More often around town should work also.
you will likley need to do more driving as the battery ages - say at more after 3 or 4 years.

I googled this:
how long does a lead acid car 12V battery hold its charge

and got this:

how long does a lead acid car 12V battery hold its charge - Google Search

www.google.com

"A SLA (Sealed Lead Acid) battery can generally sit on a shelf at room temperature with no charging for up to a year when at full capacity, but is not recommended. Sealed Lead Acid batteries should be charged at least every 6 – 9 months. A sealed lead acid battery generally discharges 3% every month. "
i suggest you do a little more reading. I don't think you need a backup battery or a trickle charger.

the worst thing you can do to a car is let it sit for long periods

 

[prairiedog]

Welcome to the forum

OK your sketch has everything wired in parallel - which doesn't work as you want but I get the idea.
It's the same need for RV's, motorhomes, caravans - a dual battery system, but only one powers the car.

What you want is an "ideal diode" module feeding the backup battery and tracker. The modules you can make or buy on eBay or Aliexpress. They are a pair of power mosfets that allow current flow only in one direction (to/charging the backup battery) with no voltage loss, but prevent backfeed from the backup battery to the main battery/car if that goes dead or you are cranking.

With the main battery disconnected the 2A charger does have full power available only for the 4.5Ah battery. Worst case charging at 0.44C but the smart charger will see the battery charge quickly and back off. You need a fuse to the smaller backup battery anyway and will have DC resistance and limit current somewhat.
Most smart trickle chargers get confused if there is a load present - some don't restart charging. Check what the Noco will do, say if the tracker draws 0.5A it might never go to float charge mode.

 

[DerryDigger]

what is too much?
or what is not enough to keep the battery charged so you don't need the backup battery?

presumably, the tracker only needs to operate when the car is operating

i expect that driving the car once a month for about 20 minutes on a highway will keep the battery sufficiently charged that you will be able to start the car. More often around town should work also.
you will likley need to do more driving as the battery ages - say at more after 3 or 4 years.

I googled this:
how long does a lead acid car 12V battery hold its charge

and got this:

how long does a lead acid car 12V battery hold its charge - Google Search

www.google.com

"A SLA (Sealed Lead Acid) battery can generally sit on a shelf at room temperature with no charging for up to a year when at full capacity, but is not recommended. Sealed Lead Acid batteries should be charged at least every 6 – 9 months. A sealed lead acid battery generally discharges 3% every month. "
i suggest you do a little more reading. I don't think you need a backup battery or a trickle charger.

the worst thing you can do to a car is let it sit for long periods

Hi WWfeldman, thanks for your response, the reason i need a trickle charger on the car is due to all the electrics that are on when car is not being used, eg: Alarm system, Tracker, Dash Cam (sensor activated). i get about 3-4 weeks out the battery before i need to jump start it. Also you must take into account cold weather, this hinders a battery also.

The reason for the backup battery, is because if car gets stolen they would take battery out of car to disable tracking system, if a backup battery was used i would still be able to track vehicle.

These are the reasons i need a trickle charger and a backup battery

 

[DerryDigger]

Welcome to the forum

OK your sketch has everything wired in parallel - which doesn't work as you want but I get the idea.
It's the same need for RV's, motorhomes, caravans - a dual battery system, but only one powers the car.

What you want is an "ideal diode" module feeding the backup battery and tracker. The modules you can make or buy on eBay or Aliexpress. They are a pair of power mosfets that allow current flow only in one direction (to/charging the backup battery) with no voltage loss, but prevent backfeed from the backup battery to the main battery/car if that goes dead or you are cranking.

With the main battery disconnected the 2A charger does have full power available only for the 4.5Ah battery. Worst case charging at 0.44C but the smart charger will see the battery charge quickly and back off. You need a fuse to the smaller backup battery anyway and will have DC resistance and limit current somewhat.
Most smart trickle chargers get confused if there is a load present - some don't restart charging. Check what the Noco will do, say if the tracker draws 0.5A it might never go to float charge mode.

Hi Prairiedog, thanks for your reply,
what worries me is that when i use an "ideal Diode" it would work great to stop backfeed, but because it wont allow backfeed, the trickle charger wouldn't know when the battery was charged (depends where i put the diode) This is the problem i need to get around. To be able to keep both batteries charged when they are not in use. If you can understand what i am saying

 

[BradtheRad]

Ideally you'd arrange each battery to be charged when it's low (below 12.8 V). To do that for two batteries, independently... is very ambitious.

It's easier if you set the trickle charge to deliver maximum 12.8 V. One or both batteries can be maintained as they require. As a battery rises to 12.8 V, the charge current tapers to a trickle automatically.

Or, install a resistor inline with the charger, of an ohm value which provides matching current level to replace current taken from the battery while the engine's off. You'll set the ohm value by watching an ammeter.

In some cases you can install one or more plain diodes in series to drop 0.6 V per diode.

All methods require time and measuring.

 

[prairiedog]

This is what I had in mind.
The car's main battery is always connected to the trickle charger. The ideal diode prevents the backup battery from ever powering the car electronics, during cranking, remote door locks etc.
A smart charger needs to see a battery at all times. If the car battery was disconnected, the ideal diode would prevent the charger from "seeing" anything and it would not inititate charging for the backup battery.
A totally dead backup battery will draw too much charging current (from the car battery) and might pop fuse F1. F2 is a fuse for the load.

You could move the diode so instead the backup battery is always connected to the charger and the diode feeds the car battery instead.

I'm not sure what you are doing when disconnecting the car battery. It's only got a few months then before it self-discharges unless the trickle charger is connected to it.

 

[KlausST]

Hi,

I see some issues with the above circuit.

You say if the car battery is disconnected:
Who disconnects the car battery while still keeps the trickle charger connected?

You say that it may blow F1.
A "trickle charger" usually provides only a little current. So if it can blow a 3A fuse .. is it really a trickle charger?

The concept of using an ideal diode misses a clear charging concept for the auxiliary battery.
So when
* a discharged auxiliary battery is installed
* a fully charged car battery is installed
* the alternator is active
.... What is limiting the charging current for the auxiliary battery?

Klaus

 

[prairiedog]

I wasn't sure what OP means with 'disconnected':
"keep the tracker with power all the time and make sure that if main battery is disconnected the rest of the car wont take power from the backup battery, (i only want the backup battery to power the tracker only)."

It seems to be in the context of thieves who have stolen the car, then park it and disconnect the main battery, yet the GPS tracker/dashcam/alarm system keeps running for a while off the aux battery?

The trickle charger NOCO Genius 2 Battery Charger is 6V/12V 2 Amp and cannot cause problems other than malfunctioning if it can't see a battery directly (so no ideal diode only is allowed). Or the two batteries have different chemistry (i.e. SLA vs flooded) it can overcharge the SLA during an equalization charge.

Yes there is a problem if the aux battery goes dead and is then connected to a healthy car system.
That should only happen if the main battery went dead, or was disconnected for a long time, which is infrequent?

Adding a resistor to limit charge current is not going to work. It's a wirewound beast that doesn't do well under the hood with vibration, moisture and the high heat can be a hazard melting harnesses and wire insulation. A voltage drop will always be there due to the tracker/dashcam current drain.
So I settled with no charge current-limiting, fuse F1 blows unless the aux battery gets special precharging treatment first.

 

[BradtheRad]

My simulation is similar to your schematic. The 47 ohm resistor can be carbon type (not necessarily wire-wound nor fragile). The diode & resistor dissipate less than a Watt in normal use.

Where there's a diode you might try an LED as a convenient indicator that connections are good. You may need to increase the resistor value as well as experiment to make certain volt and current levels are acceptable to the devices.

 

[Easy peasy]

In the ckt posted if the back up battery is a bit flat then you run the risk of blowing the 3A fuse F1 when it is initially connected.

 

[prairiedog]

Using a 3A fuse, the aux battery terminal voltage will be forced up quickly and a partially discharged 4.5Ah SLA charging current does fall back quickly as well. We dont know if the load has a low voltage disconnect, my dashcam I have it set at 12.2V so I can at least start the car. Will the aux battery ever be completely dead? A #1157 light bulb works too lol.

I did not like the resistor for current-limiting on the aux battery charging.
Voltage loss due to the loads: GPS tracker, dashcam etc. means the voltage to the aux battery is less than the car system voltage. I.e. 0.1A aux load 47Ω is a 4.7V drop which means the aux battery never charges.
Steady 3W dissipated is if there is a fault

I thought about it and a PTC resettable fuse would be better? Something that holds say 3A, trips at 6A like RUEF300. But it might get stuck, not recover if the load hogs current too when voltage is low. Some PTC fuses give the tripped resistance/holding power as a spec.