After once having run my car battery down in a camp site, I thought I would put two ODYSSEY PC1500DT batteries in. During a later adventure, my alternator died some distance from home and that second battery got me home when the main one died. But Split-charging is a complicated subject and there are loads of expensive products out there. Do you want to know how to do it really easily?
If you want a system that intelligently monitors each battery and let’s you switch the starter battery at the press of a button, you need to look somewhere else. If you just want two batteries charging whilst driving so one starts the car and the other runs other stuff (or just acts as a live spare – when a mild bit of spanner action will let you swap over if you need to), then read on!
Car batteries hold a little over 12v when healthy and the alternator pushes out roughly 14v. The potential difference means the battery will charge. (Apologies to all the electrical engineers out there for oversimplifying this, but this is a blog post rather than a text book.)
Ideally what we want is something that disconnects the second battery unless the voltage indicates the car is running. The Durite VSR above does just that.
Connect the earth of both batteries together using suitable cable. Buy one from your local Halfords or speak to your friendly autoshop if you are unsure which cable to use. You need ring terminals for these cables.
The Durite is clearly labelled, but connect one terminal to the positive terminal on the main battery and the other terminal to the positive terminal of the second battery. There is an earth connector which you need to connect to , yep, the earth of the second battery.
And you are done.
The red light comes on a few seconds after you start the engine and goes out when the voltage drops down below roughly 12.65volts, depending on the unit. If you draw power from the second battery, the starter battery is unaffected. So any accessories like fridges or mains invertors need to run off the terminals on the second battery.
Now for a little bit of optional wizardy.
If you want to have devices powered up when the engine is running but off when you are on battery, you can use the same trick. However, you want to make sure that those devices don’t draw power whilst the car is starting. The trick here is to have a VSR that trips in at a higher voltage than the split charger and a cutout at the same or higher voltage.
It is up to you if you run this feed from the starter or second battery as it will only stay running for a short period after the engine is off and is thus not really a risk to the starter battery being drained.
Durite also do suitable units, but I opted for this one as it was a different colour and thus easier to identify. I later used a labeller, which in hindsight makes more sense.
Connect the positive terminal of the main battery to the terminal on the unit. The terminal for the second battery though, connect to a suitable fuse and use the other leg of the fuse as a source of power for your devices.
I run an Amazon Echo Dot and a 4G Wifi dongle off of mine. A couple of seconds after the engine is running, Alexa wakes up.
I also use this feed to power my DRL lights, but that probably needs another post…