Lithium house batteries : Lead acid engine battery

[pauldowrick]

Hello All

We are updating my boat electrics from lead acid to Lithium Ion but have run across a problem.

Our system is as follows

House system = will be 3 x Lithium Ion with suitable Victron controllers/ chargers and alternator protection system once we go live with the new setup

Engine system = 1 x Lead acid charged straight from the alternator

Problem

There is a join switch that allows the house to use the engine battery and vice versa. It also allows the house to be charged by the alternator. This is obviously a big no as it would mean the 2 battery sets would be directly joined with the million problems that would bring.

Can anybody suggest a suitable solution.

One of my thoughts is to replace the lead acid battery with a Lithium Ion starter battery and continue as is (suitably protected alternator). Is this a possible solution?

What would be the implications of directly linking a flat battery to a full one?

Many thanks for reading this far

Paul

 

[pvb]

Why not feed the alternator's output through a low-loss splitter, which will feed both batteries but not connect them directly?

 

[pauldowrick]

Why not feed the alternator's output through a low-loss splitter, which will feed both batteries but not connect them directly?

Thats a good idea - thank you. But, we are not sure what else the join switch does. The back of it is very hidden away.

Also, would it allow the engine battery to be charged by the shore power charger

 

[trevbouy]

Two batteries of different charges will try to equalise, taking charge from the most charged one at an
uncontrolled rate.

 

[Moodysailor]

My (limited) understanding is that you need a LA battery (or similar) in the system somewhere so that charging current can be "dumped" there in the event that the BMS were to shut off the lithium bank. So for these (and other) reasons, I would stick with LA for start.

I don't know the details of what is needed for support, but I believe Victron make a Cyrix VSR that is setup for lithium banks?

I haven't looked into the details fully yet, as I did entertain the idea of lithium on our boat briefly earlier this year but the costs quickly went outside the budget so I stopped researching.

 

[pvb]

Thats a good idea - thank you. But, we are not sure what else the join switch does. The back of it is very hidden away.

Also, would it allow the engine battery to be charged by the shore power charger

Before you start spending money, you need to check what the "join switch" does. It sounds like a bit of a bodge, because the alternator charging system should ideally work without needing to fiddle about with switches.

If your shore power charger has isolated outputs, you can connect one to the engine battery and one to the domestic bank.

 

[geem]

Why not keep your unmodified alternator and set it up to only ever charge you LA engine battery. Then use a B2B Victron charger to charge you lithium house bank from your engine LA battery.
If you don't replace your existing alternator with a high output, high cooling, high temperature type from the likes of Balmar with a suitable regulator you will burn out your existing alternator trying to feed a lithium bank. The lithium will have the alternator running at full chat until its output is switched off and the lithium battery is fully charged. They are not designed to operate in this way

 

[Poey50]

As pvb says, don't spend any money. You are being tempted by a simple solution that doesn't really exist. More research is needed.

First question from me - when you say your alternator is protected, what do you mean? By an external regulator set up for lithium charging profile, by a Sterling Alternator Protect or something else?

Ah - I see geem is thinking along the same lines.

 

[pauldowrick]

Victron have battery protector system

 

[Poey50]

Victron have battery protector system

They do, but I was interested in how the alternator is to be protected.

 

[Moodysailor]

Why not keep your unmodified alternator and set it up to only ever charge you LA engine battery. Then use a B2B Victron charger to charge you lithium house bank from your engine LA battery.
If you don't replace your existing alternator with a high output, high cooling, high temperature type from the likes of Balmar with a suitable regulator you will burn out your existing alternator trying to feed a lithium bank. The lithium will have the alternator running at full chat until its output is switched off and the lithium battery is fully charged. They are not designed to operate in this way

I'm curious on this solution as it sounds elegant, so apologies if I get this wrong but isn't it preferable to have the alternator charge the lithium bank, as that way it can take in a very high current? I get that a separate alternator regulator may be preferable, but it seems to me that having a B2B charger defeats part of the benefit of lithium as the charge would be limited by either the capacity of the B2B, or the capacity of the LA bank - which is not preferable as the LA doesn't really like heavy charge/discharge cycles.

Or am I completely wrong? Thanks for your time, I'm learning about lithium setups so am curios as this was one of the areas where I deemed we would need to invest quite a bit of money.

 

[pauldowrick]

They do, but I was interested in how the alternator is to be protected.

THe battery protector limits the current that can be drawn from the alternator and passed onto the Lion batteries

 

[Poey50]

I'm curious on this solution as it sounds elegant, so apologies if I get this wrong but isn't it preferable to have the alternator charge the lithium bank, as that way it can take in a very high current? I get that a separate alternator regulator may be preferable, but it seems to me that having a B2B charger defeats part of the benefit of lithium as the charge would be limited by either the capacity of the B2B, or the capacity of the LA bank - which is not preferable as the LA doesn't really like heavy charge/discharge cycles.

Or am I completely wrong? Thanks for your time, I'm learning about lithium setups so am curios as this was one of the areas where I deemed we would need to invest quite a bit of money.

geem is talking about a reasonably common solution which avoids the expense of a new alternator, external regulator and probably a serpentine belt upgrade. It may have a role to play for those who are mainly using solar (or mains) and may be content for the alternator to make a relatively modest contribution. A 30 amp B2b is commonly used and as long as the alternator is say 60 amp or above this should work OK or at least up to a point. They do run hot and switch on an off to cope with heat so 30amps is a theoretical maximum. The advantage of this system is that should the BMS disconnect the house bank the voltage spike should be contained by the start battery protecting the alternator diodes. It doesn't however necessarily have a bad affect on the start battery - it is no different to a car battery regularly receiving 14+ volts since it will only take what it needs to stay topped up.

However, as you say (Paul Rainbow has made a similar point) I agree that this method gives up on the enormous advantage of LFP which is the ability to take charge very quickly and therefore, unlike lead acid, makes full and efficient use of the engine/alternator. Given how costly LFP is I think the single chance to do it right shouldn't be given up easily and, in the end, that is the way that I did it and haven't regretted it for a moment.

There is a third approach which is to keep a lead acid battery permanently in parallel with the LFP - this has its own complications because of the different charging profile between the two chemistries but it does solve some risks associated with sudden disconnect.

All these solutions can protect the system - with additions - against sudden BMS disconnect. However the use of drop-in batteries for all systems begs this three part question which needs to be addressed for any marine system. Apologies for repetition. You are crossing a busy shipping lane at night under engine using AIS, nav lights, and VHF. 1. How will you know if your house bank is about to be isolated? 2. What will be the consequences? 3. How will you mitigate the problems? How or whether you answer those questions determines the safety of the LFP system at sea.

 

[pauldowrick]

Before you start spending money, you need to check what the "join switch" does. It sounds like a bit of a bodge, because the alternator charging system should ideally work without needing to fiddle about with switches.

If your shore power charger has isolated outputs, you can connect one to the engine battery and one to the domestic bank.

I believe the join switch connects the two power sources together so that the engine can be started if its own battery is dead. It also allows the alternator to charge the house batteries if offshore and 240V is not available.

 

[Poey50]

THe battery protector limits the current that can be drawn from the alternator and passed onto the Lion batteries

Are you thinking about a Victron Battery Protect or a set of Victron LFP batteries together with an external Victron BMS?

 

[pauldowrick]

a set of Victron LFP batteries together with an external Victron BMS

a set of Victron LFP batteries together with an external Victron BMS

 

[Poey50]

I believe the join switch connects the two power sources together so that the engine can be started if its own battery is dead. It also allows the alternator to charge the house batteries if offshore and 240V is not available.

As said above, you don't want to be connecting a depleted lead acid start battery with a large capacity well charged low resistance LFP pack which will be sitting at up to 13.6 volts.

 

[pauldowrick]

As said above, you don't want to be connecting a depleted lead acid start battery with a large capacity well charged low resistance LFP pack which will be sitting at up to 13.6 volts.

I know. It's the reason for the original question (not put very clearly).
So, what is the solution?

 

[pauldowrick]

I suppose the quickest and easiest option is just to remove the join function. But, I like what it brings to the table in terms of security.

 

[Poey50]

a set of Victron LFP batteries together with an external Victron BMS

Ah OK. That won't regulate the alternator on it's own but it will protect the battery by isolating itself from the alternator causing a large voltage spike which will most likely destroy the alternator diodes and, if the marine electronics are on the same circuit, may cause a general expensive fry-up. Charging LFP with an alternator needs a robust alternator like a Balmar and an external regulator like a Wakespeed 500 or a Balmar MC614. A sophisticated BMS may be able to control the regulator to stop charging before a high voltage disconnect. That is the way to go if you want to alternator charge your LFP.