# What happens when two different voltages are connected to a car battery

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.

Thanks

Block Diagram:

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.

• 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. – DKNguyen Jan 28 at 5:35
• OK I will add a block diagram. – Rowan Smith Jan 28 at 5:40
• No, I need a block diagram, sorry. I have not explained it properly. – Rowan Smith Jan 28 at 5:43
• Understood..... – DKNguyen Jan 28 at 5:43
• Block diagram added. Thanks. – Rowan Smith Jan 28 at 6:02