Re: Hybrid Lead/Lithium

James Alton

I am using the Victron Super Pak Lithium batteries with the internal BMS along with a high quality German made Sonnenschien gel battery on my Amel.  I like the internal BMS in each individual battery concept better than the external because I feel it is a safer way to go.  The external BMS systems are very appealing and allow a lot more monitoring but it seems to me that there are still possible scenarios that could be dangerous that the  internal BMS option would protect against.  One scenario that comes to mind would be the batteries being charged by a temporary charging source that was not connected so that the external BMS could talk to it.  In such a scenario,  the internal BMS should disconnect the battery if any charging source exceeds a preset voltage.  Overcharging Lithium cells the main thing that you need to worry about with Lithium.  The gel battery is in parallel with the Lithiums and could handle house loads if for instance lightning destroyed the BMS's in the Lithiums or if the Lithiums we're somehow overcharged and disconnected. The gel is also a nice filter for the Lithiums and doesn't care about alternator electrical noise etc.   I personally would be leery of connecting Lithium with a common low quality lead acid battery.  What happens if you get a short in your lead acid battery and it is connected to a big bank of Lithiums?   I installed the high quality Sonnenschien batteries in boats for decades and have never had one fail in a bad way regardless of the abuse. One battery that is a similar vintage had a 1/4" hope drilled right through the case by the customer and it was caulked over, nothing came out.  The life expectancy is so long that I would often build these maintenance free batteries into the cabinetry of the boat.  These gels have virtually no self discharge and can be left without charging for up to two years so if they are healthy they will not pull down the Lithiums. They do have lower energy density than other lead acid batteries which is one downside and the cost is high.  Because the Lithiums hold about 13.2 volts during discharge the Gel doesn't really do much work unless the bow thruster is used or the Lithiums we're to ever be depleted or fail.  I look at the batteries on my boat as a critical system so I like having the gel.battery backup. One could lose their boat if power failed at the wrong time after all.  By the way, the Sonnenschien gel that is installed on my boat was purchased from a customer who wanted his 8 hear old Sonnenechien batteries changed ahead of a circumnavigation. (which he completed.)  That battery is now well over 20 years old and still seems healthy though given the age I want to replace it with a new battery.  So those are my thoughts and current solution. This is of course an evolving technology.   James Alton SV Sueno

On Thu, Dec 16, 2021 at 5:40 AM, Dean Gillies
<stella@...> wrote:

On the subject of safety ... 

A couple of years ago when I started thinking about a hybrid solution, I frequently heard the risk statement "you can't mix chemistries" because apparently bad things will happen. (I have a cartoon picture in my mind of a mad professor dropping lumps Lithium into sulphuric acid in a beaker producing lots of coloured smoke lol.)

I have never seen any explanation of what that actually means.  What exactly is the worst bad thing that happens when you 'mix chemistries"?  I would love to know.

There is an electrical interaction between the battery types, and it's important to understand how that can play out in worst-case scenarios. What else?

My safety assessment was focussed on risks which are peculiar to lithium and an interconnected hybrid architecture. There are plenty of risks with lead batteries which are also extant in lithium batteries. These are important, but are not reasons to discount LFP. 




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