Originally Posted by albergsailor
so using a echo or duo charger is the way to go then. That does sound quite alot easier and better. sorry for not specifying. What i had planned on doing was having 2 deep cells on one bank and 2 high cca starting batteries on another. Having a normal switch appealed to me in that it just seemed more reliable then some type of solenoid or the like that automaticly monitered the batteries. I will check out the echocharge or duocharge but im still upset that this has been such an issue to just find a decent on/off switch that specified make before break. The product that defjef uses seems pretty nice. thanks so much for all the advice i will be looking into everything and i will let you know how it all turns out
Let's take these in turn.
1. EchoCharge or DuoCharge.
These are voltage follower devices. They are not combiners and they are not relays and they are not isolators. What they do is sit between the house and starting battery banks and, whenever they sense a charging voltage (i.e., about 13VDC or above) on the house batteries, they bleed off some amperage to top up the start battery. The EchoCharge can handle up to 15A. The DuoCharge can handle up to 30A and is programmable. For most installations, the EchoCharge is more than enough. When you start your engine, you typically only draw less than one amp-hour
from the start battery (lots of amps, but only for a few seconds). That energy is replaced in a very short time.
2. Battery bank configuration.
Normally, it does not make much sense to have more than one start battery. You're just wasting space and $$$. You can hook up your system so that in the event of a start battery failure you can simply use the house batteries to start. Much better to use the space for an additional house battery, in many cases.
3. Battery types.
Yes, by all means you want true deep cycle batteries -- not "dual purpose" -- for the house bank. 6V golf-cart size batteries in series are often the best choice for a house bank, as these have thicker plates and handle the deep cycling better and longer than do comparably-sized 12-volt batteries. And, you do want a true starting battery for the engine. On most boats a group 27 is plenty, or a group 31 if you want a bit of overkill. Flooded batteries are by far the best price-point. AGMs have their place, but have significant problems, too (you've got to be sure they're fully charged at least once every week or two). Also, several users have reported catastrophic failure of AGMs. Gelled batteries are often a very good choice, though pricier than either flooded or AGMs. In particular, they're good for boats kept on a mooring
, and those in weekend-mostly operation. They are very long lived, compared to both flooded and AGMs.
4. Make-before-break battery switches
. They're easy to find. However, IMHO it's not a good idea to trust them to protect your alternator, since an inadvertent switch to the OFF position -- and in the absence of a separate wire to the alternator field -- could wipe out your alternator's diodes instantaneously. In any case, if you wire the DC system correctly this is a moot point, since; (1) the alternator output never goes thru the switch; and (2) there's no need to switch anything anyway, since the EchoCharge or DuoCharge handle things nicely and automatically.
Again, IMHO, these devices are to be preferred to battery isolators and charge combiners, relays, etc., since they do not directly interconnect battery banks which may be of very different capacities (e.g., a large battery bank and a small start battery) and at very different state of charge levels (which could result in high inrush currents and, possibly, to the near depletion of a fully-charged smaller battery into the much lesser charged house batteries.