I have purchase a battery charger solution that has a two part system. I am having a few problems with it which has led me to realise that the unit supplies two different voltages to the battery at any point in time.

The system has two units, they are both connected to the alternator and the battery in parallel.

The first unit is effectively a solenoid, it delivers up to 120Amps directly to the battery. It does not change the voltage, whatever voltage the alternator is outputting gets delivered to the battery.

The second unit is a "smart charger" DC/DC converter, it takes the alternator input and converts at the output to a particular charge profile. This can be up to 14.7 volts depending where in the charge profile it's at. It will pass a maximum of 20Amps to the battery.

So together the two units can deliver 140Amps. Of course this is limited by the amount the battery bank can take and the size of the alternator. MY alt is 115Amp, the most I have ever seen going into the battery is 63Amps which teters off quite quickly.

Apart from the fact that the two devices are connected in parallel they have no way of communicating with one another.

My question comes about because I always understood that you couldn't connect DC sources of different voltages in parallel.

So if my alternator is outputting 13.8 volts (which it seems to do all the time) and the DC/DC converter is outputting 14.7 volts, which it will do at some point in it's charge profile, what voltage does the battery see and how can this even work?

The product in question is a CTEK250 and CTEK120 for further reference.


Block Diagram:

enter image description here

The earth cable from the SmartPass is tiny (0.05mm2??), while the positive cables need to be 35mm2. The cables to/from the DC/DC smart charger are all 4mm2.

  • \$\begingroup\$ Solenoids are loads. They deliver nothing to the battery. Since you have misused the term "deliver" for solenoids, I cannot trust what you say about the smart charger since I cannot tell if the smart charger is taking power from the battery and charging something else, or if it is taking power from something else (like the alternator) and charging the battery. Most of this would be cleared up with a block diagram. \$\endgroup\$
    – DKNguyen
    Commented Jan 28, 2020 at 5:35
  • \$\begingroup\$ OK I will add a block diagram. \$\endgroup\$ Commented Jan 28, 2020 at 5:40
  • \$\begingroup\$ No, I need a block diagram, sorry. I have not explained it properly. \$\endgroup\$ Commented Jan 28, 2020 at 5:43
  • \$\begingroup\$ Understood..... \$\endgroup\$
    – DKNguyen
    Commented Jan 28, 2020 at 5:43
  • \$\begingroup\$ Block diagram added. Thanks. \$\endgroup\$ Commented Jan 28, 2020 at 6:02

1 Answer 1


When the SmartPass conducts to allow current from the alternator to flow directly into the house battery bank, it shorts out the Smart Charger disabling it. This is similar to placing a copper wire right between the input and output of the Smart Charger. This forces the Smart charger to basically turn off. Note this is different from a short-circuit where you connect the output of a power source straight to ground. This is a short across a device which electrically removes the device from the circuit.

Put another way, electrons take the easiest possible path. If you provide an alternate path that takes practically zero effort to traverse, practically all electrons will take that path and practically no electrons will take the other paths leaving the devices in those paths without power to operate.

So in your scenario at least, you are not actually connecting different two voltage sources together (unless you consider the output of a functioning battery charger or alternator being connected to a battery to be that which it is. But you already know what happens there. The larger voltage wins and shoves current into the devices with lower voltage)

  • \$\begingroup\$ So if the SmartPass never turns off the SmartCharger will never charge the battery. So the SmartPass needs to "disengage" when it detects that the current has dropped to some value so that the SmartCharger can start doing it's thing? \$\endgroup\$ Commented Jan 28, 2020 at 6:21
  • 1
    \$\begingroup\$ @RowanSmith Yes, more or less. The Smart Charger when conducting is basically so low resistance it steals all the current that would otherwise go into powering the SmartCharger so the SmartCharger can't actually turn on to do anything. Even if it did turn on (i.e. maybe it has an external power source to run its internal mechanisms but still works by drawing on alternator power to convert it, it wouldn't be able to since the input is directly tied to the output. It would be like you trying to move your car by pushing it from the inside. \$\endgroup\$
    – DKNguyen
    Commented Jan 28, 2020 at 6:23

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