Mixing AGM and wet cell batteries

[Jonathan316]

Small sailboat already has a new 80amp wet cell. Now the owner wants to add an Optima D34M AGM battery to the mix, giving him about 200 amps total.

I know you cannot hook dissimilar battery types in parallel, but is it OK to mix AGM and wet cell batts when using a Off-1-2-Both battery switch? When switched to ''Both'', are the batts effectively in parallel?

Also, he has a 9amp feed from the alternator on his small outboard which skipped the electrical panel and hooked straight to the wet cell for charging. How can that still be used in the new setup?

Any advice would be appreciated. I cannot talk him out of choosing between one or the other batts; he is set on using them both for his house system. I know I can isolate each bat completely but would prefer to figure out some sort of mixed system.

[Silmaril]

The charging requirements of the two batteries are very different. The charge, acceptance, and float of these two would almost require two sepearate charging systems. A charger optimized for the wet cell will be too high voltage for the AGM shortening it''s life, and one optimized for the AGM won''t keep the wet cell up to optimum charge, shortening it''s life.

[BeneteauMark]

When a battery switch is set to ''Both'' the batteries will be paralled. Bad idea due to different electrical characteristics.

As far as reusing the outboard alternator, same problem as with a regular battery charger. The alternator is undoubtably engineered for a wet cell battery, and will not be compatible with the AGM.

[928frenzy]

I''m about to add a deep cycle wet battery as the starter. The existin deep cycle AGM batteries will become the house bank. The charger is set at 13.6 volts, which (I''m told) is ideal for AGM bateries. The charger will also be hooked up to the starter battery. However, since it will also get charged by the engine''s alternator (at 14.2 volts), it seems to me that it should get an adequately charge every time I run the engine to leave and enter the dock and marina. Usually, the engine is on for about 30 minutes to get to decent sailing areas. Also, once the engine is turned off, I will switch to the house bank for power. I don''t intend to use "Both" unless there''s an emergency.

Will this setup be ok?

~ Happy trails and sails to you ~ _/) ~

[PaulBl]

A deep cycle battery as a staring batter is not as good as real starting battery.

You will get get more cranking amps from a starting battery than a deep cycle. You really don''t ned a deep ccyle batter for starting. I use deep 2 cycle AGM''s as a house bank but a regular sealed starting battery for starting. I can combine both for an emergency.

[drwill]

928 frenzy: As long as you use separate chargers (e.g., alternators) with separate voltage regulator for each type of battery, you can use different battery types. It''s OK to use the "Both" battery switch position for emergency power, but you must never use "Both" when recharging.

Will Burton S/V Far Niente

[928frenzy]

Thanks for the reply. I returned the deep cycle wet battery for one that had more cold cranking amps, but not deep cycle. I''ll install it tomorrow as my starter battery "only", unless there''s an emergency.

The new charger has a feature that allows for three stage charging (which charges in spurts up to 14.2 volts). The charger is presently charging the AGM batteries at 13.6 volts. I plan to connect the starter battery to the charger. Should I install the three stage module?

~ Happy trails and sails to you ~ _/) ~

[928frenzy]

After two weeks with the new charger, all seems well. The starter (wet) battery has more than enough charge (14.2V) to easily turn the engine over, while the house bank AGM batteries are kept charged at 13.5V.

The switch is placed in "2" (the starter battery only, the engine is started, we pull away from the dock and motor to the sailing ground. We put up the sails, shut down the engine, move the switch to "1" (house bank) and sail away. On return, we reverse the process. So far, so good. ;^)

~ Happy trails and sails to you ~ _/) ~