Technical - Solar power Round 2

I'm taking advantage of our technical/real-life stopover to do what I was mentioning in the last Solar Power blog post. We haven't had any issues with the solar power so far, the lowest it's ever gone was 76% when we pulled on it quite a lot with the water pump, the laptop charger and while we were parked in the shade. However, as the Stark say, Winter is coming and with it, shorter sunlight hours and less direct exposure. Because of it, I started being a bit concerned about our mobility. As you remember from the previous post, the battery that is connected to the solar controller and that we use to power our stuff is also the battery we are using for the engine, which means that if we discharge it a bit too much, we could end up being stranded. The battery that we have on the engine has quite a high CCA (cold cranking amps) (750A) but a small capacity (50Ah) which is fine for a starting battery but for a leisure/accessory battery it's not ideal. You want high Ah. With a 50Ah you can pull 120W for 5h, with 80Ah, for 8h. There's no scenario where we can expect to pull 120W as we don't intend to run any kitchen appliances but you get my point.

I digress. Because of this, I decided to install a second battery, that one with much smaller CCA and much higher capacity. They can be a little tricky to find because they're not sold at your usual car battery store, I ordered mine from Amazon, it was delivered in a staggering 24h and weighs a cosy 20kg. It's a Hankook 80Ah DC24MF deep cycle specifically intended for RV/Leisure usage. I wanted to have both batteries connected so I could change over from one to the other in case of an issue so I added a switch to allow for continuous changeover, they're not cheap but in the even of a discharge or issue with the main starting battery it means i can potentially use the other one as a backup or use the solar panels to recharge it. 

The switch

Important point: This is not a dual battery setup in the strictest sense of it. In order to connect two batteries in parallel and have them continuously connected to one another you need to add what is essentially a charge controller (VSR - Voltage Sensitive Relay). The device will look at both batteries charging states and decides which one needs charging and which one doesn't. I decided against it because since we have 2 batteries and 2 sources of charging, things get a bit complex and in our case unnecessary since in normal situation, the engine battery gets charged by the alternator while the engine is running and the accessory battery gets charged by the solar panels. Every other case is considered a "limp mode".

Simplified wiring diagram

The diagram is pretty straight forward, note that the link between the batteries and the MTTP controller is a bi directional link, it can both charge and discharge. The ground is supposed to be in common, Ground DC in a car is always a bit tricky because everything is relative (since a car is isolated from the outside world). What you want is to have the least amount of resistivity between the grounds of both batteries. usually the battery of the engine is grounded to the engine block through ground straps (sometimes excessively).

More straps brah'?

Some street racers say you can get a more stable engine if your ground is "like totally good".  The chassis of the cars are usually connected to the ground as well. Anyway, the mttp controller has a common ground and I used a terminal to put all the other grounds in common and then connected it to the bolt connecting the driver seat to the chassis. We measured less than 1 Ohm resistance between that point and the engine battery ground. Then again our ohm-meter is cheap and for that kind of resistances I wouldn't put too much trust in that measure.

The ground terminal (they didn't have black cable)

For this installation, since we won't see high currents (>20A) anywhere i went with 16mm². It's a bit overkill but it seemed necessary for the battery terminals.

Finally the last aspect was to connect all that to the engine battery. Going from the cabin into the engine bay is always a fun endeavour since the firewall is supposed to be airtight & water tight. Thankfully, Renault's engineers put the fuse box just on the driver side, with an enormous penetration into the firewall, a little pushing and pulling and a little bit of grease later and we were through, into the fusebox.

The result is pretty clean, I'm really happy with it, much better than what the previous had done, he had run the cable below the car, attached with a succession of zip ties. beautiful.

Finally, i've stored the battery under the driver seat, first of all, because there was space, second of all because it's 30cm away from all the connectors and switches. 

Picture taken before the seat was re-installed

Closing note: As I've mentioned before this is not a conventional dual battery setup or a two-battery storage system. There are a lot of things in this setup that would make it a poor system for those two above applications. Batteries need to be similar to operate continuously in parallel and I don't mean similar as in same-ish, they need to be the same capacity, the same brand, the same age. So here we have the option to put "Both"on the coupler, this is only to be used in emergency as it could damage both batteries. Because batteries behave like vases and we've all done this experience when we were kids.

So if you plug two batteries at different states of charge, the charge will want to equalise, so you get current flowing in your wires, except that batteries are capable of producing very very high currents (750A for my engine battery). Quick calculation, for a cable having a 1 Ohm resistance:

P = r.I² = 1 x 750² = 562 kW/s 

Enough to make a nice cosy fire.

It won't happen because the batteries have internal resistances that increase with heat and that amount of power would definitely heat up a bit (it would actually break/explode)... In any case, my point is that is it extremely dangerous and ill advised to go to "both" position if the batteries are at very different states of charge.