9. When to Use a Dual-Purpose Lithium Battery
By the Blue Heron Tech Support Team
Each battery is generally designed for a particular purpose, which may make it less suitable or not even usable for other applications. Battery applications can include House or Storage, Automotive or Starting, Portable Communications, Emergency Power, Electrical Vehicle, and more. So, it is helpful to understand the application a battery is intended for and match that up with our needs. We’ll talk about two common uses: House Storage and Starting.
· A Deep Cycle House battery is a “Storage Battery” that can hold a large inventory of power and discharge that power at a low to moderate rate for a very long period before needing recharge. Such a lithium battery is best based on prismatic cells, and would have a BMS that allows a moderate discharge rate, perhaps up to about 1C (e.g., 60A normal and max. 100A for a short period discharge, for a 100Ah battery).
· A “Starting Battery” is one that requires a very large discharge of amps for a relatively short period of time, to start some piece of machinery. That demand could be in the 200A-250A range for smaller internal combustion engines, or 400A-500A.
Let’s say we have a smaller sailboat that does not have a separate house battery and starting battery. Such a boat could be a 30 to 34 ft. sailboat, or a 20 ft. center console boat. Radios, running lights, depth sounder, fish finder, wind instruments, and maybe some smaller refrigerator, trolling motor, fans, etc. can be found on or both of these type boats as their house loads. And, a 100Ah lithium can serve those needs splendidly. But, in addition that battery would need to start a small, let’s say 25-HP diesel auxiliary on the sailboat, or maybe 115-HP four-stroke outboard on the center console. So, it would be great if that same battery had the ability for the burst of power needed to start such engine sizes. So that is where a dual-purpose battery fits in nicely.
Another example would be in our Class C motorhome, which has a small generator to charge the House Battery Bank. The Battery bank when outfitted originally with AGMs has been used as the Starting battery for the generator. The chassis or cab battery for starting the motorhome engine is separate from the House battery bank. So, in converting to lithium, we need a battery that can handle the starting loads of the generator as well as the House loads such as 12V refrigerator, lights, fans, sound system, and, inverter for AC loads. So here too is where a nice dual-purpose battery fits in nicely.
The Blue Heron 105Ah Dual Purpose is rated for Deep Cycle, but also includes an upgraded BMS, that can supply a 250A burst of power for 10 seconds of continuous cranking to start an engine. After a short rest one could do another 10-second cranking burst. We’d recommend trying less than that. And, if your engine doesn’t start in that shorter period, give attention to other issues with the engine, like fuel and ignition.
If you just try a regular lithium battery intended as a Storage Battery to also be a Sarting Battery, you may find that the Lithium Battery BMS just trips off. Because the BMS is sized to limit the level of Amps, and you are proving it is too low for your starting purposes. We have tested our Dual-Purpose battery on 50-HP diesel auxiliaries and generators, and up to 115-HP outboards, and it functions fine for that purpose, plus supplying house loads. It also has a handy Voltmeter to observe battery condition.
Some have asked us about using this Dual-Purpose battery as a starting battery for the main engine on their yacht, as they have Blue Heron lithium batteries for their House Battery bank. We have recommended to them we feel the better approach would be to use AGMs as the starting batteries, as the lithium setpoints for chargers and alternators will work satisfactorily on AGMs. And, there is not the benefit to having the added Ah’s of lithium for this application.
A motorhome or small boat that needs a larger House Battery Ah capacity than 105Ah, or more than 250A of Starting power, can connect two of these Dual-Purpose batteries in parallel, and have 210Ah of House battery bank power, and close to 500A of starting capacity!
(This technical article is the property of Blue Heron Battery LLC and may not be copied or reprinted in whole or in part without the express permission of Blue Heron Battery LLC.)
