A journey to B2B and back again, by way of the Ring.

[Derekoak]

When I built my first off grid camper. Batteries were lead. Lead lasted longest if fully topped up continuously. Simple split charge systems common at the time did not do that very well, so I bought a B2B a Ctek ds 250S. Combined smart charger from alternator and solar mppt. Only 20 amp maximum but it kept my 80 amp hour battery in good condition and ran a compressor fridge and other lesser loads.
When I built my 2nd camper lithium was possible but too expensive so I got a temporary 80AGM battery and a Ring Rscdc 30, Planning a lithium when I had run the agm into the ground. The Ring seemed to be like the previous ctek but had 30 amp maximum. It was reasonably cheap and effective (when it worked). Sometimes it stuck and did not charge when the engine started. I took it back to Ring HQ in Leeds a couple of times and they got it going again and Phil @Admin described the reboot sequence, but the Ring was never reliable.
Then Phil and @Harryw went for cheap Chinese Lithum cells for DIY battery making. I decided to tag along. My 2nd camper had space underneath for the battery but not for the usual rectangular battery shapes. I needed squarer and DIY allowed me to do that.
Phil suggested that under the right circumstances Lithium was possible to split charge rather than charge by B2B. This was contentious. His argument was that lithium did not need to be topped up to 100% ASAP for health in fact it was better for it to stay at around mid charge. The expense of B2B was to my mind mainly to ensure full topping up. He showed a system that charged from the alternator with a Victron cyrix li ct. Most others wanted to stay with B2Bs mainly because there were videos put out by Victron showing smoking alternators caused by uncontrolled charging.
@trevskoda said that his system was even simpler just a manual switch to trigger a power relay to join leisure battery to starter battery when he decided. He said alternators were under most load at startup so he switched his switch after a few minutes.
I did a test temporarily joining my starter and 120amp alternator to my lithium battery at 12.9volts with a 16mm copper cable. At idle there was 59amps at 2000rpm 66amps. I decided that I was unlikely to damage my alternator and copied Phils system.
I got my extra complications sorted, heater circuits for the lithium cells when cold, especially needed with an underslung battery. Since then my system has been working well. In fact too well! My 200ah battery is nearly always full! This is maybe not good for it. So recently I installed a switch in the cab switching the cyrix signal wire. So now:
if I am not using my doblo as a camper (fridge off) I have this switch off and the alternator does not charge the lithium, (I have a bypass wire to switch the signal wire on when the ignition is off so that my solar can top up my starter battery)
If we are camping but solar is good the switch is off if the lithium SOC is below 75% as experience shows that my 160watt solar can keep up with our demand in good conditions. This must save a bit of diesel.
Finally, copying Trev, the switch is off for the first few minutes after starting to reduce strain on the alternator even further.
So this means that the switch is off more than it is on. I do wonder whether the cyrix Lict was unneccessary too. If the switch is fed with ignition positive even if you inadvertently leave the switch on when you stop the engine, the leisure battery still disconnects from the starter battery when the ignition is off. The cyrix may be cheaper than a B2B at 80 pounds, but a power relay only costs 25 pounds.
Anyway for me a B2B has been shown to not be needed with my Lithium.

 

[wildebus]

I go for a B2B with Lithium still. Not because of the Victron Video showing a smoking alternator, but as a way to manage the maximum charge current that is pulled from the Alternator. The Labour cost of a Garage to replace the alternator, plus the Alternator itself is a fair bit more expensive than the cost of a B2B, not to say the extra inconvenience if it happens.
I (and others also that I know who do similar things for people with campervans), have had the situation where the increased load of charging a Lesiure Battery has tipped the alternator over the edge and have had to say to the owner they need to get it fixed/replaced, and that was with a 100Ah Lead Battery. Put a larger Lithium into the equation, with its ability to take a much greater charge and for a longer period and you run the risk of a continuous high load over-straining an Alternator that is likely not speced up for that level of running. So the chance of that less-than-perfect alternator installed under the bonnet failing is reduced.
FWIW, I have my own B2B set so one half of it will not turn on until 20 seconds after the engine is started. Then the other half will not turn on until 20 more seconds so it is a staged load. And the Lithium will disconnect from the B2B (or any charger for that matter) when it hits 14.1V so it will not be getting a constant charge.
Good luck to anyone just running a simple relay setup - It should work fine, but for myself I prefer to go for the more cautious approach of current-limitation which also includes automation (I would never remember to turn the switch on and off as the appropriate times so I let the technology do that for me )

 

[Derekoak]

I agree with you wildebus. Doing work for others you have guard against things going wrong way down the line. You may still be blamed.
However I know many people who believe in b2bs to such an extent that they install 2 times 30 amp. That is the same Amperage as I am drawing. My Amperage falls a good bit as the lithium voltage rises. The 2x b2b may still both be trying to draw max bulk current for longer than me. Their cost of installation is 5 times mine and for me I do not see the advantage.
What I do know is that thanks to your sale of a victron 75 15 smart mppt to me, my 160amp semiflexible solar, my cyrix lict and my 200ah lithium I am very flush with offgrid electricity.
Because I have spare capacity my Bms is switching off charging at 13.9volts at the moment. As I just don't need the capacity 99% of the time.
The switching protocol I described in the first post is somewhat complicated to fully automate. It includes what purpose I have for my car tomorrow or the next few weeks with different switching for different uses, as well as a SOC based protocol and a ignition delay sequence that can probably be automated.

 

[wildebus]

Interesting the amperage falls as the voltage rises for you - on my mains chargers I have found the current is full on to the limit of the charger from the minute it goes on until the point the battery is full and it cuts off the charger itself (I actually reduce the charge current setting on my Multiplus from the maximum to 85% so it is not running at full capacity for potentially hours - lower stress on devices is always good to have). I wonder if what you are are getting is a feature of the BMS? Not saying it is wrong or a problem, just different to what I have noticed on commerically packaged Lithiums.

The Solar and B2B are a bit different as the solar harvesting fluctuates naturally and alternator output can drop due to engine speeds so hard to make a good comparison there.

 

[Derekoak]

I have 2 different bms one on each 100ah x 4 cell. So what I see from a standard sayv13volt lithium start watching one bms only is a jump to around 28amp then a fall as the lithium quickly gets up to a voltage of 13.3 or 4 to say 23 amps then a continuous fall to my cut off voltage of 13.9v finishing at around 12 to 15 amp. Of course the other 100ah is doing a repeat alongside. Although one likely finishes before the other.
It seems sensible to me. The alternator is wanting to put out up to 14.25 volt and 16mm cable and the lithium have some back voltage and back resistance. As the back voltage increases it is obvious that a simple system will just reduce the amps flowing? When I had the ring the alternator voltage was somewhere up there at first and the ring was doing its "smart" control for my Agm and its 30 amp quickly faded to low values single digits, I assume because that was what an Agm needed.
I have not tested the voltage at the starter with my new system as charging progresses.