Minimum needed to convert to lithium?

[wildebus]

I understand what you're saying WRT using a B2B and you're right of course but I was considering what would happen with a straight swap to lithium with no modifications to the existing charge circuit, ie drop in lithium.
Yep, the current is flatter for longer with Li but the available voltage from the alternator is essentially boosted by the B2B in order to enable this. Drop in doesn't have the same advantage and your left with I=V over R and I can't see how the current can possibly go up when the voltage differential goes down.

you are making an assumption aboiut what is fitted and I think basing your thinking as if you had a cable from starter battery to habitation battery. There is usually on a motohome a low-current device connected by a thin cable.
You end up at best with an existing setup that is running at its full capacity for lomger than it is designed for and run the risk of it failing due to exceeding its duty cycle (problem 1) and then not outputing the voltage required anyway (problem 2).

Vendors promote Lithium as Drop-in. I just don't agree and that is my opinion on it. I think I have had this conversation with someone before on this forum and he had a go at me for not providing a dissertation for my reasons.
All I will say is each to their own but I would not reommend a Lithium Battery is fitted to a Motorhome with a charging system that is barely adequate for a Lead Battery whos charging needs are way lesser. If you asked me to fit a Lithium Battery to your motorhome with that setup, I would just not do so.
I would suggest rather than basing your calculations purely on Ohms Law for a theroretical answer, you need to consider what is physically installed and how they can be impacted (and the cost of repairs when things go wrong) due to overloading.

 

[barryd]

This thread is making my head hurt.

I dont think I want lithium anymore.

 

[Nabsim]

Lol Barry, the way you stay static you could disconnect main and engine charging and run from solar initially. Sites you use don't normally have hook up do they?
Separate charger would sort mains charging anyway, it doesn't have to be all done in one go

 

[barryd]

Lol Barry, the way you stay static you could disconnect main and engine charging and run from solar initially. Sites you use don't normally have hook up do they?
Separate charger would sort mains charging anyway, it doesn't have to be all done in one go

Yes we do tend to spend three or four weeks in one place off grid when going to our favourite haunts. I generally disconnect the cab battery as it runs flat otherwise. Not sure it will be that easy or advisable on a newer van though. Dunno. I agree, it can all be done in stages. If we get a new van we may do a bit more touring anyway and move around more but I love the way we travel now. I like to be in one place for a good while.

 

[Fisherman]

This thread is making my head hurt.

I dont think I want lithium anymore.

you started it

 

[barryd]

you started it

Nope not me! It was Egg head Geoff. Rob and me chipped in but quickly lost the will to live.

 

[Fisherman]

Nope not me! It was Egg head Geoff. Rob and me chipped in but quickly lost the will to live.

Doctor David is prescribing a full lithium set up to bring you back to life Barry.
As for Rob a few beers will bring him back to life

 

[merl]

In any series arrangement, current is limited by the device that will pass the smallest current. As an analogy, consider a water pipe with an orifice plate. Without the orifice plate, the pipe can pass a lot of water; with the orifice plate, the flow rate is restricted by the orifice plate. Now in our alternator charging system we have four components (OK, five including the chassis for earth return):

• the alternator;
• the split-charge relay;
• the wiring; and
• the battery.

The wiring is rarely the limiting factor, which means that it's either going to be the relay, the alternator or the battery. Most alternators fitted are capable of much more than they typically supply, and so are also rarely the limiting factor. Split charge relays are usually designed to offer as little resistance as possible and so are also unlikely to be the limiting factor.

This leaves the battery. Now lead/acid batteries have a relatively high 'reverse' internal resistance -- IOW (in other words - not Isle of Wight, Rob!) while the battery presents little resistance when supplying current, it has a relatively high resistance to current in the opposite direction; IOW it acts a bit like a diode! On the other hand, the resistance of a LiFePO4 battery to a charging current is much less. It still might be the limiting factor, but the current limit will be considerably higher than it is with lead/acid -- unless, that is, it has current limiting built into its BMS. Of course, you could arrange things so that the wiring is the 'orifice plate', but the voltage drop across, and hence the energy dissipated by, the wiring will be relatively large, which could lead to the wiring overheating with potentially disastrous consequence. Even if the BMS has current limiting, you have a single point of failure that could cost you your motorhome! At least, that's AFAICT!

Some might question why I posted the OP if I could work out all the above: Simply, I hadn't thought it through and this thread has prodded me in the right direction; and so my thanks to all. Secondly, even if I'd worked out all the above, people (like David) who handle this stuff professionally, might know a thing or two that I'd completely missed (e.g. is there a current limiter that can be fitted in series to allow using the alternator to supply the same order of charging current as it does to a lead/acid battery). My thanks to all.

Hi Geoff, what you are alluding to with your orifice is the resistance part of i=v/r.
We don't use JUST the single largest restriction (the part with the highest resistance) but we add ALL of the resistances together (because they're in series) so we have to add ALL of the resistances in the whole charging circuit and that includes EVERYTHING even each rectifier diode within the alternator, every piece of wire every contact and add the lot together and we have the R in our i=v/r calculation.
A 30A fuse alone( not counting the resistance produced from the metal to metal contacts is nearly as much as a LFP battery and there's probably more than one, then all that cable, then all the contacts and connections, then the relays etc etc etc.

you are making an assumption aboiut what is fitted and I think basing your thinking as if you had a cable from starter battery to habitation battery. There is usually on a motohome a low-current device connected by a thin cable.
You end up at best with an existing setup that is running at its full capacity for lomger than it is designed for and run the risk of it failing due to exceeding its duty cycle (problem 1) and then not outputing the voltage required anyway (problem 2).

Vendors promote Lithium as Drop-in. I just don't agree and that is my opinion on it. I think I have had this conversation with someone before on this forum and he had a go at me for not providing a dissertation for my reasons.
All I will say is each to their own but I would not reommend a Lithium Battery is fitted to a Motorhome with a charging system that is barely adequate for a Lead Battery whos charging needs are way lesser. If you asked me to fit a Lithium Battery to your motorhome with that setup, I would just not do so.
I would suggest rather than basing your calculations purely on Ohms Law for a theroretical answer, you need to consider what is physically installed and how they can be impacted (and the cost of repairs when things go wrong) due to overloading.

David, As I said, just questioning your reasoning behind your post saying that current would increase with drop in lithium, not necessarily advocating it.
I don't have a standard wired motorhome with a lead acid battery and a lithium battery I can swap it for so can't do a real word swap and see what happens. So no other way of hypothesising except got using Ohms law

Doctor David is prescribing a full lithium set up to bring you back to life Barry.
As for Rob a few beers will bring him back to life

Doctor David sounds a little grumpy today, maybe it's because of all those extra hours and the additional work that the government have set without a relative pay rise.
Or maybe it's twats that come to his room and keep asking daft meaningless hypothetical questions?
Best shut up before I'm banned from the surgery.

 

[Fisherman]

Doctor David sounds a little grumpy today, maybe it's because of all those extra hours and the additional work that the government have set without a relative pay rise.
Or maybe it's twats that come to his room and keep asking daft meaningless hypothetical questions?
Best shut up before I'm banned from the surgery.

Doctor David has to be one of the most patient folk I have ever known.
As for twats asking hypothetical questions, I myself have probably fallen into that category lately. But I never question what the Doc says, he knows best.

 

[Nabsim]

Ah but that's the thing. What is best is not always within peoples reach, there are many things in between lol

I don't always agree with what David says but I don't think I would ever disagree on technical points or best practice. I am certain he wouldn't give bad advice and I would recommend him to anyone. I still won't always agree with him (or anyone else) though lol

 

[wildebus]

Ah but that's the thing. What is best is not always within peoples reach, there are many things in between lol

I don't always agree with what David says but I don't think I would ever disagree on technical points or best practice. I am certain he wouldn't give bad advice and I would recommend him to anyone. I still won't always agree with him (or anyone else) though lol

I don't agree with myself half the time

Right now trying to work out the best layout for a bunch of kit to go on a van wall and keep disagreeing with myself where to put things

 

[Bigshug]

Right now trying to work out the best layout for a bunch of kit to go on a van wall and keep disagreeing with myself where to put things

I’m pretty sure you will win the argument

 

[wildebus]

I’m pretty sure you will win the argument

yeah, but probably won't talk to myself afterwards for a few days

 

[davef]

After getting advice on here I replaced my split charge relay with a B2B charger, using a Renogy 20A one, much cheaper than the Victron and seems to work fine and has the added advantage that it allowed me to fit a switch on the dash to limit the charge to 10A, which I use after start up until the starter battery is full. Currently available at £80.99 - https://www.ebay.co.uk/itm/12499774...6&customid=hCpTEf0eAAAAj2gYJ4VeDpxDFeYPAAAAAA