Replacing lead acid habitation batteries with Lithium

[motorhomeninja]

I'm looking at the Frogstar web site and it states that their lithium batteries with a BMS, battery management system, can be a direct replacement for lead acid or AGM habitation batteries provided the charging profiles for the solar and mains charger meet the higher voltage needed for the lithium batteries. Split charge relay and any battery master can be left as is. Has anyone done this and did it all function ok? I know the more recommended route is a B2B but the above is within my technical abilities! (I think?)

[colin]

On wildcamping.co.uk there are several members who have replaced lead acid with lithium, in past drop in replacements haven't been 100% successful, not sure if this is still the case.

[stevec176]

Last year I enquired with a company advertising drop in replacement Lithium batteries if they were compatible with my Rapido 665f and after supplying details of my system the reply was no. Ended up being too costly as most of my system would have had to be replaced, so not just drop in then.

[arthur49]

I would not risk it without a B2B and appropriate cabling/fusing.  If your van has a smart alternator how is the voltage/charge rate going to be controlled?

If you want to maximise the life of your lithium battery, alternator, charger, control unit and cabling, install properly with a B2B etc so you are presenting the battery with correct voltage/charge and let the BMS be a last resort protection.

Sorry, to me, I think its marketing bovine effluent to "drop in replace"

(I have two LiFePo4's and LOVE them.  We very rarely have EHU.  I would not go back to lead-acid so I am a huge fan of LiFePo4 ....... but this "drop in" nonsense .......)

[MikeF]

Battery sellers are not going to suggest there may be issues using their batteries, however Fogstar batteries are high quality and are recomended. 

It's  often suggested lithium needs a 'special charge voltage, this is not true. A 12v lithium battery will happily charge to full, at charger voltages as low as 13.8 volts. Victron specify  14.2 volts charge and 13.4 float as the default  lithium garage voltage.

However the battery charging sources must safety  deliver the possible high charge currents that lithium batteries  may accept.  Unlike lead acid batteries,  that to some extent, self limit the charge current, a lithium battery, when charging, will take is the maximum that the charge sources can deliver. This may stress some charging systems. What is important with lithium is not to 'overcharge'. If held at high voltages higher than its 'full resting' volts of 13.4, for long periods,  then accelerated  ageing  takes place. This may shorten useful life to a few years. 

The function of a lithium battery BMS  is often misunderstood.  In no way does it control any charge process. It's purpose is to protect the battery and it does this by effectively  disconnecting from any charge or discharge path if safety limits are exceeded .  Idealy  the charging and discharging voltages  and currents should be inside the BMS protection  'envelope'.

How does this effect a typical motorhome electrical system?

Split charge charging system.

This simply connects the starter battery and alternator to the leisure battery. Very high currents may flow, only limited by the circuit resistance  between  the alternator and lithium battery ( due to the very low Internal resistance of the lithium battery).  This could result in currents 50 to over 100 amps. This could be bad news for the alternator and anything between it and the lithium battery.  There is the added issue that there is no termination of the charge process.  As discussed lithium does not like being subjected  to a continuous  high voltage whilst fully charged. 

The advantage of using  a B2B charger, I recomend the Victron Orion b to b 1230, 30 amp charger,  is that almost all existing installations  will not be  over stressed by the 30 amp charge current,  and the unit has a ideal charge profile for lithium.

The Victron can directly replace a split charge relay  with minimal circuit changes.

The use of a battery master or similar will be slightly less effective,  depending on charge voltages,  but will still be useful as a maintraining charger.

Mains or solar chargers MPPT type, can be used with lithium provided,

There is no temperature compensation ( only needed for lead acid).

The charge  voltage, ( absorbtion voltage) is 14.4 volts or lower ( ideal 14.2)

The absorbtion period is less than 60 minutes ( ideal 30 minutes)

The float voltage is 13.5.

Solar chargers, the low cost PWM type are not recomended and may cause damage to lithium batteries .

There is other information regarding installing lithium batteries that may be discussed as the thread continues . 

Mike

[motorhomeninja]

Many thanks for the input. B2B it is then.

Graham

 

[arthur49]

"A 12v lithium battery will happily charge to full, at charger voltages as low as 13.8 volts. Victron specify  14.2 volts charge and 13.4 float as the default  lithium garage voltage."

Mike what is your view I've heard from one LiFePo4 manufacturer that the battery may charge at below 14.4v to full, but cell balancing will only occur at 14.4v - 14.6v ?

[MikeF]

Re  cell balancing.

Battery  management systems, BMS, tend  to start cell balance when the cell is over 3.4 volts, 13.6 for a 4 cell 12v battery. Any inbalance will only show up when cells are almost full, this is voltages  over 3.4 volts per cell when under charge.( a resting, no charge or load, full battery will have a voltage of 13.4 to 13.5 volts.

I also have seen the manufactures data you refer to and I think its a translation error.

I may be wrong but a BMS operating on this principal, only activating balance at 14.4, would be a poor design

Mike