Here is another 'hack' I have been working on, somewhat reminiscent of the DIY Lithium Battery. This time the goal is to get fully configurable bluetooth enabled battery to battery charging for $30.
Cliff Notes: Add a 12V-24V boost DC-DC converter between the truck battery and the solar panel input of an MPPT charge controller - charge controller will convert the 24V down to the correct voltage to charge your battery.
A couple of important caveats:
- This is a 'hack' and not supported by Victron, myself or anyone else, try it at your own risk and only if you know something about electronics - you could break something.
- This will only work with an MPPT charge controller with a common ground, do NOT try this with PWM charge controller, it will end poorly.
- I have tried this and verified it works with one particular charge controller (Victron 100/20) and a generic DC-DC converter, it will likely work with others, but proceed carefully.
I have a 160w solar panel and a 150ah LiFePO4 battery, so far I have done fine relying primarily on solar to keep the battery charged and meet my needs. My camper is connected to the truck with a Blue Sea Battery Link ACR (stock FWC) through 6AWG wire in the truck to the stock 10AWG(?) in the camper. Between that and the Toyota 'smart' alternator I only get ~5A charge current while driving. While I don't need more charging, I have thought about increasing the charge current while driving just to have some redundancy. So far I haven't jumped on commercial B2B chargers for a couple of reasons - they are expensive for what they are, the one programmable one is even more expensive, and they are unidirectional - you can only charge the house battery from the truck battery. My truck/camper is an adventure mobile only and sometimes sits for long periods, so having the camper solar charge the truck battery is important.
The Victron MPPT charge controller is essentially a buck only computer controlled DC-DC converter. It really doesn't care where the input voltage comes from, just that it is at least a few volts higher than the output voltage and can source the load current. The idea here is to step the 13.xV from the alternator up to 24V using an inexpensive DC-DC converter, then feed that into the MPPT charge controller to step down to the ideal voltage for charging my batteries. There are several pros to this:
- The MPPT 100/20 is bluetooth enabled, completely programmable and already programmed to coddle my fancy battery, including a low temperature cut off.
- I already have the MPPT controller, and the DC-DC converter is only $28 plus $2 for a switch,
- I can wire it up so there is a switch for unidirectional battery to battery charging while driving/camping, then switch it over to parked mode where it goes back to being bi-directional and charging my truck battery via solar.
Besides the caveats listed previously, the biggest con to this is that you need to have a specific type of ACR and charge controller for this to work. In addition to the charge controller being MPPT, to use a cheap non-isolated DC-DC converter is must also have a common ground. This means that the solar panel -ve terminal must be tied directly to the battery -ve terminal. This is the case with the Victron 100/20 and is likely the case with other Victron units. I don't know if this is the case with other charge controllers, but you can check with an ohm meter. The other requirement is that if you want this to be automatic, your ACR must not have an under voltage lock out. When the ACR is connected to the DC-DC converter it won't see any voltage on the house battery, so it will kick in the low voltage lock out and not engage.
The diagram below shows how to wire this up and the required parts (in the dashed box) are:
In my testing, I have seen a solid ~19 amps going into the battery, drawing ~11A @ 24V from the DC-DC converter and about 20A from the alternator. With the toggle switch in the 'up' or 'in use' position it works like any other B2B charger, with the toggle switch in the 'down' or 'storage' position it works like the stock ACR set up.
Edited by rando, 11 September 2019 - 03:55 AM.