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Starter Lead Acid House Lithium (Battery Query)

Hi All,

Quick question. Does anyone here have information about using two types of batteries, one for start (Lead Acid) and another (2) for House (Lithium) Do I need two chargers? What about the 1 | 2 | Combine switch is this still usable? I like the idea of the CCA of a lead acid for the starter battery as we have an ancient bukh dv24 diesel that takes a bit of kicking to get going. I am afraid it would harm the lithium bank if I went fully lithium. Any suggestions will be greatly appreciated.
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Re: Starter Lead Acid House Lithium (Battery Query)

The two chemistries definitely need different charge profiles.


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Re: Starter Lead Acid House Lithium (Battery Query)

We have been using LFP house with LA start for over 3yrs. We chose the simple path of putting a Blue Seas ACR between the house and start and just combining the batteries. The ACR will leave the batteries combined full-time until the LFP voltage drops below 12.7V - at which time the ACR disconnects the house and start.

In this manner, the start battery will always be held at float voltage (13.3V) when a charge source is not present, and will charge up to the LFP maximum voltage of 13.8V when a charge source is present. If the LFP house bank gets below the LA fully charged voltage of 12.7V, then the ACR disconnects the start from the house and leaves the start isolated at full charge.

Any downside to this method is that the start battery is "technically" not getting its proper bulk/absorption charge profile. However, the start battery is never discharged more than a few Ah's during starting, and that is immediately recovered, so it never experiences discharge cycling.

I suppose one could argue that keeping a LA battery on float almost continually, and only occasionally charging it to slightly less than designed bulk voltages, will shorten its life through sulphation.

This may be the case, but our start batteries have been working like new for over 3yrs, and they are just $50 Walmart car batteries. So if they only last 6yrs instead of 7yrs, they are cheap and easy to replace, and we aren't concerned about the slight lifetime hit.

Your other option is to use an expensive DC-DC charger to provide the "proper" charge profile, or to use a second alternator dedicated to the start battery.
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Re: Starter Lead Acid House Lithium (Battery Query)

The manual combine switch would keep the batteries in parallel like the ACR, only it will not disconnect them when the house falls low. You could do this manually like you have been, but it is a complication. So it can work - but better to get an ACR for this job.

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Re: Starter Lead Acid House Lithium (Battery Query)

Also, the lithium bank will outperform any LA battery for providing high current for long time. Your old Bukh would love it.

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Re: Starter Lead Acid House Lithium (Battery Query)

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Originally Posted by colemj View Post
Also, the lithium bank will outperform any LA battery for providing high current for long time. Your old Bukh would love it.

Mark
We really do need more information such as the brand, model and type of LFP batteries you are considering. Marine specific LFP batteries can easily start engines in which case you could keep the lead acid as a reserve or for starting and charge it with a DC to DC battery to battery charger such as a Sterling Power BB1230 from the LFP bank where all charging would be routed to. Many in drop-in LFP batteries can't handle high cranking or other high load current, due to the mosfet based BMS switches current limitations so you would not want to crank the engine with this type of LFP battery.

The cranking limitation is not a limitation of the cells inside but rather the sealed internal BMS design that uses FET based switches. Most of these BMS's/switches can trip on over-current and a high BMS temp (over-heating caused by drawing or charging with too much current.)

In early September a transient customer who had a yard in RI install a bank of drop-in batteries lost all electrical power while docking and nearly took out a 7 figure boat in 20 knots of wind. No one had bothered to look at the BMS current handling capability of the drop-in LFP bank. When he hit his bow thruster it worked for a few seconds, then poof, no 12V power. When he finally got tied up, the batteries eventually re-set, but his alternator was toast...

Marine specific or DIY LFP banks, that use actual contactors, such as Mastervolt or the Lithionics OPE-Li3 system can handle starting, windlass, thruster larger inverter etc.. loads with ease. The new Lithionics drop-in model 12V125A-G31-5CND-LRB can also handle cranking..

One drawback to an ACR is that it is bleeding LFP energy to float the LA battery. If you have excess energy not a big deal but if you don't then every little bit helps.

We also do not yet know where the forthcoming ABYC standard will land on paralleling Lead and LFP, so a DC to DC charger is not necessarily a bad idea, if your vessel is insured and you may need a compliant LFP system.

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Last edited by Maine Sail; 4 Weeks Ago at 07:52 PM.
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Re: Starter Lead Acid House Lithium (Battery Query)

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Originally Posted by Maine Sail View Post
One drawback to an ACR is that it is bleeding LFP energy to float the LA battery. If you have excess energy not a big deal but if you don't then every little bit helps.
Yeah, I wasn't considering the plethora of "drop-ins" with their severe charge/discharge limitations.

Our paralleled start battery is currently drawing 170mA as the solar has it at 13.4V on its way to 13.8V - which isn't onerous for us, but could be for some. I'd say that anyone installing lithium shouldn't plan the system so that a 2-4 Ah/day draw puts them on the edge.

Besides, it goes the other way too...you get it back when the LFP draws below 13.2V.

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Re: Starter Lead Acid House Lithium (Battery Query)

By the time regular ACRs isolate, the LFP bank would be dead flat, not a great scenario.

What you need is adjustable setpoints.

Have a look at the Magnum Smart Battery Combiner.

Adjustable setpoints, I believe "connect" as high as 13.8V, and LVC at 13.5V would work well to prevent Starter from drawing LFP House down when a charge source is not active.

25A limit so not to be used for jumpstarting, and as usual best for all significant charge sources to be routed direct to House, so the combiner is just used for maintaining Starter.
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Re: Starter Lead Acid House Lithium (Battery Query)

With todays modern protection systems, overcharging, low voltage etc what is the benefit of a separate start batt..la or otherwise
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Re: Starter Lead Acid House Lithium (Battery Query)

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Originally Posted by RegisteredUser View Post
With todays modern protection systems, overcharging, low voltage etc what is the benefit of a separate start batt..la or otherwise
Many boaters today, and many who don't know a lot about LFP, are looking at buying "drop-in" LFP batteries. These batteries have often have, especially when choosing based on price, low current handling FET based BMS switches that are not intended nor designed to handle high current such as starting loads.

We have a reader of MarineHowTo.com who bought himself a "Sweet 300Ah LiFePO4 battery". Problem is he had no idea what he was doing and his 300Ah LiFePO4 battery has a max charge rate of just 50A. He blew up a couple of alternators and had numerous BMS disconnects before he emailed us. His entire system had to be reconfigured around his dainty little BMS and in the end he was charging slower than flooded lead acid batteries...

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