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Best way to charge vehicle battery

I have a van install with solar MPPT charging when it's sunny, EasyPlus Inverter Charger charging when I am connected to the shore power, and B2B charging from the alternator when driving. So that takes care of the leisure battery. But how can I best charge the vehicle battery when I am parked up without shore power? I wonder if I can use a second B2B configured in the opposite direction (they are designed to be used in either direction but obviously not one single unit at the same time). Excess solar energy would be drawn from the leisure battery (I have used this method previously but at that time I didn't have a B2B from my alternator to charge the leisure battery). If not, I have also looked at Battery Maintainers. I am concerned about what happens when it is sunny (so there is solar MPPT charging the leisure battery, the vehicle is running with the alternator charging the leisure battery, and the solar/leisure also charging the vehicle battery through the second B2B or the Battery Maintainer. Furthermore, I have Battery Protects on both the charging and the load sides of the leisure battery and I have read that there can be issues with reverse current (but I can't get my head round where this kicks in). Despite being an electronics engineer by 4 year apprenticeship and understanding Kirchoffs Law of current at a junction, I am very concerned about the possibility of doing something stupid which could jeopardise my very expensive van install (all Victron-based including a £2K Lithuium battery). Can anyone help with any degree of certainty please?

battery charging
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2 Answers
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dazey77 answered ·

I have a small 40w portable array (2x20w) with a controller that I can use to boost the main solar charger for leisure battery or charge the vehicle battery. Was the best solution I could find and it costs me nothing as I had the mobile unit anyway.

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Hi, thanks for the reply. Do you also have a Battery To Battery charger charging the leisure battery from the alternator when the engine is running. The issue isn't how to charge the vehicle battery standalone; the issue is when you have a full system with charging capability in both directions. The issue occurs when charging from one size raises the voltage sufficient to turn on the charging from the other side. That would/could lead to energy lost when the total inefficiency of the two chargers is greater than the excess energy being delivered by the solar panel. I know I can add the second B2B if I switch off the other chargers but I am looking for a simple solution which doesn't rely on switching. I have been told by a Victron distributor that they have been trying for years to find a solution to this problem!

I have a cyrix-li-ct between the engine battery and the leisure battery. It will only engage when the engine is running (so will never go from leisure to engine for example). It also has a cut off switch, so in summer its disconnected 99% of the time.

I guess if the portable solar was plugged into the engine side then it could techically trigger the cyrix and also charge the leisure battery, but in reality it will never do this as the solar array on the leisure is much larger and its lithium so the voltage is higher.

Yep, it really does come down to the voltage trigger levels and I guess it will only be through trial and error that I will find out the art of the possible. Thanks once again for your reply.

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Alexandra answered ·


This is quite neat little charger doesn't draw much power and keeps the vehicle battery topped up. You could always set up a relay to switch it on automatically if you were looking for an no think setup. Not sure what mppt you have but some allow a streetlight function that can be adapted.

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Thanks for the reply. Unfortunately I am looking for an off grid solution, so not mains powered (not even inverter mains either). I have been told by Victron distributor that they have been looking for a solution to this problem for years. The issue is that, without switching of some kind, it isn't possible due to A charging B charging A issue - where the losses in the chargers could be greater than the excess energy from solar.

On the overlanders they just use a small solar panel with a small pwm charge controller dedicated to start batteries.

That's as simple as you can get.

Hi, thanks again for the reply. Do you know if they also have the vehicle battery/alternator driving into a B2B as per my system? The issue is that when the solar is sufficient to start charging the van battery, the voltage is high enough to enable B2B charging of the leisure battery. But with the inefficiency of the B2B plus any energy going into the leisure battery, this is enough to start draining the vehicle battery. This is the problem. I am looking for a solution which covers a full system, i.e. with solar charging the leisure battery, alternator charging the leisure battery plus a way of charging the vehicle battery without causing a drain on one of the batteries.