B2B charger (sort of!)

Mark135

Mark135

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I have a Bailey Autograph MH that has a split charge (or VSR) relay to charge the LB's. I have fitted a pair of 100ah LiFePO4 batteries and want to charge them quicker when the engine is running. I have a stand alone 40a mains charger which I intend to use off my inverter when driving using the vehicle (starter) battery. I will wire and fuse this and use a 200a relay to ensure it's only powered when the ignition is on. I will manually change the inverter battery connections with a 175a Anderson connector so either the vehicle battery or LB's are connected at a time. The mains charger connects to the LB's through the other side of the Anderson plug so it's impossible to use the LB's to try and charge themselves.
Sorry, a bit long winded but I think this will function similarly to a B2B albeit with some efficiency loss. My question is: Do I need to disconnect the existing SCR/VSR or is it OK to run both at the same time? The SCR is supplying a constant 14.2 v (measured at the cab 12v socket), the charger peak voltage is 14.6v
Thanks in advance.
Mark
 
I would disconnect your split charge/VSR because the charger will not only feed your leisure batteries but feed back through the link to the engine battery. In effect it will be powering itself. Presumably your 40A charger has a LiFePO4 charging profile but if it is also powering itself and the engine battery it will never know what the state of charge is. It might raise the vehicle voltage from 14.2V to 14.6V and I have no idea how the alternator and ecu will react to that.
 
Okta
My 40a charger is a lithium profile. I was struggling to get my head around how the charger would behave, as you suggest. I think the vehicle ecu/alternator will be ok with the slightly higher voltage as it potentially already has that from the solar charger when the the scr is active, but determining the SOC may be problematic with a lead acid and lithium battery in parallel and the charger may not function as expected. Reading a few web sites about retrofitting battery to battery chargers, the ones that give enough detail say to disconnect the scr so that is what I'll do. Thanks for the help.
 
Trouble is if you disconnect the VSR you also disconnect the fridge. The distribution of fridge and battery is done on the circuit board in the Power distribution unit so not easy to do at all
 
On my CBE unit I put a D+ triggered heavy duty relay on the connection from the distribution board to the leisure battery. This left the fridge working on 12V but isolated the leisure battery from the engine battery when the alternator was charging. I have no idea what you would need to do on a Bailey.
 
PeteS said:
Trouble is if you disconnect the VSR you also disconnect the fridge. The distribution of fridge and battery is done on the circuit board in the Power distribution unit so not easy to do at all
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Is it possible to simply disconnect the feed from the vsr/Power distribution unit to the LB and leave the fridge connected?
 
Okta said:
On my CBE unit I put a D+ triggered heavy duty relay on the connection from the distribution board to the leisure battery. This left the fridge working on 12V but isolated the leisure battery from the engine battery when the alternator was charging. I have no idea what you would need to do on a Bailey.
Click to expand...
This sounds like a good option. I've ordered a 100a 5 pin relay so I can fit it "normally closed". Just got to work out which cable to fit it to🤔. Thanks.
 
No because it’s all integral on the pcb. I think as Okta said use the D+ line going into the Pdu to disconnect the leisure battery from the pdu when the engine is running.
however I don’t think you need to, the volt drop on the D+ line due to current trying to flow down it won’t have any effect on the B2B at the vehicle end, plus the fridge will also be dragging it down.
I personally would just try it.
on mine when solar is at its peak producing 11A at 14.4 volts with the engine running, soon as the fridge is turned on it drops to 13.5v.
your problem might be trying to isolate the B2B from the fridge to stop the B2B from powering the fridge
 
PeteS said:
No because it’s all integral on the pcb. I think as Okta said use the D+ line going into the Pdu to disconnect the leisure battery from the pdu when the engine is running.
however I don’t think you need to, the volt drop on the D+ line due to current trying to flow down it won’t have any effect on the B2B at the vehicle end, plus the fridge will also be dragging it down.
I personally would just try it.
on mine when solar is at its peak producing 11A at 14.4 volts with the engine running, soon as the fridge is turned on it drops to 13.5v.
your problem might be trying to isolate the B2B from the fridge to stop the B2B from powering the fridge
Click to expand...
Thanks Pete,
I've ordered a relay to isolate the LB from the vsr when the engine is running but as you suggest, I might just give it a go without to see if it works. The only issue I can see is how the charger behaves. If it works I'll have a spare rely from AliExpress. As an aside, according to my Bailey Autograph manual, the fridge is connected directly to the LB, it says not to switch the fridge to battery when the engine is not running or it will drain the LB.
 
easy way to prove the fridge part is what does the battery voltage do on the panel when you turn the fridge to 12v? If it is connected directly then the voltage will drop reasonably quickly.
 
Because I have a b2b charger, a disconnected the SCR. My fridge is AES, but is always powered from the engine battery. The load when parked is minimal, the load when driving is from the alternator.
 
in h said:
Because I have a b2b charger, a disconnected the SCR. My fridge is AES, but is always powered from the engine battery. The load when parked is minimal, the load when driving is from the alternator.
Click to expand...
So you have the fridge on the battery alll the time there is no gas or hookup? Surely that must use a lot of battery power unless you actually leave the gas on virtually all the time?
 
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PeteS said:
easy way to prove the fridge part is what does the battery voltage do on the panel when you turn the fridge to 12v? If it is connected directly then the voltage will drop reasonably quickly.
Click to expand...
Just tested my fridge. I have a shunt on my LB so I looked at current on my LB's.
Engine off, fridge on battery, negligible current draw.
Engine on, about 5a
Engine off, ignition on, negligible draw.
The only thing that remains on for all cases is the fridge interior light which is a small led.
My conclusion is there is a d+ activated relay on the fridge 12v element connecting it to the LB's and the manual is talking rubbish.
 
I've just fitted a 40A relay to disconnect the battery from the PDU driven by the D+ line, it works really well, fridge works as usual via the pdu, battery is charged by the B2B.
I have also fitted a switch in line to the D+ for that occasion when I need to charge the Leisure battery vis the B2B but still need the lights on. ie parked up. That works well too
 
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