# Robust low battery cutoff

I am building a 5V DC charger for an offroad motorbike. The main goal is to use this to charge/power a mobile device/phone/tablet whilst travelling/camping. If space allows maybe also a GPS tracker but that's another subject for another day...

The circuit will be powered by the lead acid motorbike battery. With the engine off the battery measures 12.8v. Engine on between 13-14v

The Regulator I have chosen is a SKM10A-05 for a number of reasons (temperature range, heat dissipation, size etc...) but mostly due to the input range due to engine cranking. This also simplifies the design and as a one off i'm not too concerned about price.

To prevent accidental drain of the motorbike battery I want to implement a low voltage cutoff. At around 11v I want to turn off the SKM10A-05 using the R.C. pin:

FUNCTION REMOTE CONTROL;

1. Power OFF: R.C. ~ -Vin <1.2Vdc or short
2. Power ON: R.C. ~ -Vin >5.5~75Vdc or open circuit

I find plenty of topics for lower voltage applications or charging a battery but so far nothing for this voltage range. Preferably this circuit also needs to not consume much power and be quite robust as the bike gets thrown around alot.

Would really appreciate any help!

• I’d be concerned about only having 25 volt surge rating. – Andy aka Apr 29 at 19:29
• "this circuit also needs to not consume much power" Could you please specify "not much power" in Watts? – Huisman Apr 29 at 19:35

OK, this is a fun one to noodle around with. Assuming that the R.C. pin on the regulator is pretty high input impedance, this might be one way to do it if you get a kick out of building from basic components:

simulate this circuit – Schematic created using CircuitLab

This circuit consumes about 20 uA. The turn-off threshold is controlled by choosing the R1/R2 voltage divider, which turns off Q1 when the Q1 base voltage falls below the base-emitter diode drop. This, in turn, happens when the battery voltage falls below some value (11.4 V, in this case). The threshold is a bit sensitive to the R1/R2 values, so you might either want to add a small series trimming pot, or at least another smaller value in series with one of them to fine tune the threshold.

With these values, the threshold is around 11.4 Volts. Simulation output is below, with voltage on the vertical axis. The battery voltage swept from 11 to 12 volts (blue trace) and the output (orange trace) taken from the collector of Q2. You might want to add an aggressive low-pass RC filter on the output. Sorry for the tiny fonts on the graph....don't know if I can change that on CircuitLab. You can see a reasonably sharp jump in voltage output as the battery voltage crosses 11.4 V.

The turn-off threshold will have some non-trivial temperature sensitivity, but it might just be tolerable for this application.

If you do not mind making a circuit to handle this, I could propose a circuit that has a maximum 1mA leakage current. You could use a Zener diode to create your reference voltage and compare the existing voltage with this reference, using an op-amp. The output of this opamp could drive the gate of a P-MOS or anything else like a Relay.

Look for supervisor IC's, e.g. the TPS3711 from Texas Instruments. You can use the Typical Application given on the first page and leave out the pull-up resistor at the output.

The IC itself consumes only 7 uA, the preceeding voltage divider should be designed such to optimize power consumption and noise immunity.

The suggested IC can handle up to 40V, but, like Andy aka is commenting, you likely need to protect the SKM10A-05 against voltage spikes from the engine. This protection can serve the supervisor IC as well.

• Thanks, I really like the low consumption! I struggle with the design of circuits. Think I will add a P-Mosfet also to prevent reverse polarity. So I guess just a low pass filter before both the IC and the regulator? – Sentien May 10 at 12:31
• @Sentien Why not use a polarized/keyed connector as polarity protection? The regulator doesn't need a low pass filter, this supervisor IC needs a decoupling cap of 10 nF. You do need a circuit to protect against high voltage spikes > 25V – Huisman May 10 at 18:17