Mini AGM parallel with lithium

[bloodman]

Don, you made this statement and I am very interested
"If the Lithium EXCEEDS the engine requirements, the AGM does not necessarily have to be large enough to function as a stand-alone cranking battery. I have seen several AGM's from smaller battery grounds (one in particular used an ATV battery) that worked very well."
I currently have 2 lithium starters parallel through a selector switch for starting, electronics, ect and was thinking of adding a small AGM as a "shock absorber" to the system.
From your statement on another post I assume this is acceptable.
Thanks

[bloodman]

I guess if you trust all the chinese BMS out there, OK. I've personally had several over voltage events. One saved by a circuit breaker and the next saved by me and voltage alarms.
The AGM is a safety cushion just in case.

[EuropeanAM]

Cleaned this thread up to remove the non-relevant engines/comments.

In your case, a smaller AGM (that met even half the engine requirements) could function to stabilize voltage profiles and provide a fail-safe voltage supply in the event of BMS shutdown.

Although I personally would just go with an AGM from the start, I've seen a couple of boats using 24 series AGM's in a hybrid bank, and at least one where a powersports battery was placed into a hybrid bank.

[bloodman]

Thanks

[SGrem]

Is this a motor issue or a battery issue? I Captain a ton of boats with these motors and many of the owners are going to lithiums....but if they need to just stay with AGM i would like to understand how best to advise folks i know and different owners. Thanks.

[EuropeanAM]

While Mercury has stated that Lithium Batteries that meet certain specifications are permitted to be used, they are not "recommending" them. Among the reasons would be:

1. Engine's programmed (Idle Charge Management) Strategy is calibrated for AGM battery voltage profiles. This system is standard on ALL V6, V8, V10 and V12 Mercury 4-stroke Outboards, and is designed to monitor battery voltage/condition, raising the engine RPM when engine is at idle (IN NEUTRAL) if it senses the need to increase output. Most Lithium batteries have a voltage profile that differs significantly from the programmed strategy.

2. BMS (Battery Management System) functions completely separate from the engine's PCM, and can literally take the battery offline when it perceives the temperature is too low, too high, high demand, and other factors that vary from one BMS to another. Taking the battery completely off line results in several potential risks/scenarios:

A: If the engine is decelerating or at low RPM's, there is a POTENTIAL that voltage could drop below the minimum threshold for relay operation. Engine dies.

B: Loss of voltage doesn't work well with an ELECTRONIC THROTTLE AND SHIFT engine. These items (and thus the battery) are not "optional accessories".

C: (Most Common): Alternator goes to FULL FIELD condition, in an effort to charge a battery that it senses as 0.00 Volts. ***

*** When this occurs, voltage climbs SIGNIFICANTLY above 12V (often in the low-mid 20's at idle). PCM will reduce RPM's rapidly/significantly due to the high voltage, and there is a significantly higher chance of PCM and Component damage due to incorrect voltage, especially when on plane where voltage can climb well north of 30VDC.

The "hybrid bank" (or better yet, just sticking with an AGM) significantly reduces the risk of these possibilities.

[SGrem]

Gotcha. Great info as usual. Thanks for that excellent explanation.

[bloodman]

It is the "C" scenario that I believe happened to me. First time the breaker went off. Second time I saw it happening and got some power sucking things going and saved it. It is also what I believe some others have described happening to them. Great explanation and very understandable. This is what I want the mini in the bank for.