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Author Topic:   Dual Batteries-Wiring Different Types
Fischer posted 03-16-2004 02:17 PM ET (US)   Profile for Fischer   Send Email to Fischer  
I just looked over the reference section on wiring dual batteries to one engine. It says avoid using different types of batteries. The problem being the lower charged or deepcell will pull down the engine cranking battery.

Fishermen tend to use the two types to take advantage of the deepcell for their trolling motor. Is there a good way to wire two different types and still use the engine to recharge both batteries or is it better to go with two dual purpose batteries?

tlynch posted 03-16-2004 04:05 PM ET (US)     Profile for tlynch  Send Email to tlynch     
Check this out:


prxmid posted 03-16-2004 05:32 PM ET (US)     Profile for prxmid  Send Email to prxmid     
I'm in the process of doing the same thing and have gotten 4 different answers to this question
jimh posted 03-16-2004 05:46 PM ET (US)     Profile for jimh  Send Email to jimh     
There are really two parts to the question:

--Can batteries be paralleled and charged effectively;

--Can batteries of different types be properly charged by an outboard engine's charging circuit.

My views:

I don't think it is a good idea to parallel batteries for regular operation or charging, even ones of the same type, brand, age, and condition. Even worse is paralleling batteries of different type, brand, age, and condition.

I don't think that outboard engine charging circuits can optimally charge all different types of batteries even individually, and of course not in parallel.

But it will work; the batteries will get charged!

Chuck Tribolet posted 03-16-2004 10:09 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Some place I saw a reference to a battery charger that input
a nominal 12V (i.e something like 9V-15V) and output the
right stuff to charge a battery.

I have a similar widget on my whaler. It's a Humminbird
SureVolt that inputs anything from 6V to 24V and outputs 13.0
V at up to 1.5A IIRC. It runs my fishfinder and GPS and they
don't shutdown when I crank the engine. It doesn't put out
enough current to run my VHF (6A on transmit).


Fischer posted 03-16-2004 10:37 PM ET (US)     Profile for Fischer  Send Email to Fischer     
Does anyone have experience with a battery combiner, do they work?
What about dual purpose batteries, Do they perform as a deep cycle and cranking battery or just a sales pitch. Deep cycles and crankers are two different animals and it's hard to beieve they can be combined and service the draw of a 12 volt trolling motor.
jimh posted 03-16-2004 10:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Fish--there must be a web sites that goes as deep into batteries as we go into Boston Whalers.
Chuck Tribolet posted 03-16-2004 11:02 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The dual-purpose batteries are a compromise, but an acceptable
compromise in many cases. They aren't a perfect starting
battery, but when a group 24 starting battery starts the high
compression 350 cu. in. V8 in my 'Vette just fine, I don't
need a perfect starting battery to start the little old 90HP
Evinrude on my whaler.

But they aren't inteneded to run a trolling motor all day and
THEN start your outboard to go home. If I had a trolling
motor (I don't fish), it would have its own deep cycle
battery. The dual-purpose batteries are great for running
your electronics for a while at anchor.

The dual-purpose batteries can be deep cycled, at least a
few times. I had the trim-tilt switch fail closed in the
garage, and the battery spent some considerable period of time
trying to jack the transom off the trailer. When I discovered
the problem, the battery was DEAD. I don't mean low, but
DEAD. The voltage was like four volts. I put it on a charger,
it came back, and ran the boat for another year.


Chuck Tribolet posted 03-16-2004 11:09 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
One more thing: If I had a trolling motor and a deep-cycle
battery to run it, I'd have a way to use the deep-cycle
to start the motor in a pinch. It might well be a set of
jump cables I carry in the console (which have more than
paid for themselves earning beers jump starting buddies).

I did this once on a rental houseboat. Something ran down
the starting battery while we were beached for a couple of
days. We'd run the genset regularly so the house battery
was well charged, so we just jumped it to the starting
battery and it fired right up. BTW, the houseboat was only
about five months old.


Whalen posted 03-17-2004 01:25 AM ET (US)     Profile for Whalen    
Chuck is right on the money on the dual purpose batteries being a compromise. They can do both deep and crank, but will not do either as well or as long as a dedicated batt. I have just installed a battery combiner between my crank and deep batteries. I really like it. I am talking about a true combiner, which is properly called a separator. It uses a switching relay and logic circuit to determine when to combine and separate the batteries. I looked into the stealthcharger, but since my needs are at 12 volts, I did not need the expensive unit. I ordered a Surepower model 1315, about $90.00, and am very happy with the quality. I have also seen a very slick unit at west marine that does the same thing. Do not consider a battery isolator that uses resistance and diodes, all wrong for this type of charging. I put my GPS on the crank battery, and the sounder on the troller battery, and set both of them to display the battery voltage. I can see exactly what is going on charge wise. I use the trolling motor (12v) all the time, but since I average 50 miles each time on the bay, I keep the battery happy. The 1315 can be used to charge both batteries also from a external battery charger hooked to either battery. I just plug a charger into the trolling motor plug. I am working on a circuit to charge from the towing vehicle also.
Moe posted 03-17-2004 08:48 AM ET (US)     Profile for Moe  Send Email to Moe     
It does NOT hurt to parallel two different sizes/types of batteries WHEN CHARGING off an alternator. The alternator is controlled by a voltage regulator that keeps the output at a fixed voltage (if the engine is turning fast enough). That's all the voltage regulator does. The current through each leg in a parallel circuit is independent of the current in the other leg(s). Each battery will draw the charging current it needs depending on the applied voltage and its own state of charge. The main problem with paralleling batteries when not charging is that if one battery develops a shorted cell, it will drain the other battery down. That's why those with a OFF-1-2-BOTH switch should probably never run it in BOTH if there's a chance they'd forget to take it out of that after charging.

For starting, you may want a Group 24 AGM battery. For trolling, you want the largest flooded-cell battery you can fit, Group 31 if possible.

You can use a battery separator (relay-type) to connect the trolling battery to the motor when the alternator is running.

You can also use a voltage sensing relay from BEP and add there master cut-off switches for safety. Here's the info:


Fischer posted 03-17-2004 01:57 PM ET (US)     Profile for Fischer  Send Email to Fischer     
Thanks for getting back. I think I'll go with the combiner, deep cycle and cranking battery. I didn't think tha combination battery would performe like a deep cycle on a trolling motor. I've got the 1,2,both switch installed so I'll just add the Combiner. Thanks again
jimh posted 03-17-2004 08:18 PM ET (US)     Profile for jimh  Send Email to jimh     
Many exotic batteries require particular charging cycles, so I don't know if a constant-voltage charge is the optimum for all types of batteries. This is really the topic for a web site that specializes in batteries.

In a parallel arrangement of batteries, given a constant-voltage charging source, each battery absorbs current independently, that is, each battery acts like the other battery is not there provided the charging voltage is truly a constant voltage. If the current being drawn by one of the batteries influences the charging voltage, then you cannot say that the batteries absorb the charging independently. Well, again, this splitting of hair is more for discussion on web site forums that want to delve into the minutiae of batteries and how to properly charge them.

I still think that having two batteries of a similar type, brand, age, and condition is a good way to go if charging them in parallel and from a not particularly sophisticated source of voltage and current (which is how I would classify an outboard motor).

Chuck Tribolet posted 03-17-2004 10:36 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Moe: Why should the trolling motor battery be AGM, and the
starting battery a flooded type. AGM batteries are for
preventing acid leakage. They are for severe
environments with lots of strange G forces, like F-16
fighters, motorcycles, and whalers taken into REALLY nasty
conditions. And offshore racers, and maybe offshore sailors.
Either you are in those conditions, and BOTH
batteries should be AGM, or you aren't, and neither should
be AGM.

I've had my Montauk in some ugly conditions. I once had
a LOT of hang time at six knots (huge wind chop). I've seen
Admiral Linda get the whole boat and most of the prop out
of the water jumping the Monterey Express's wake. There's
never been any evidence of acid leakage from my flooded

Jimh: Constant voltage is OK, but often (maybe always) non-optimal. If the voltage
is too high, it will eventually wreck the battery. If it's
too low, it will undercharge the battery. So GOOD chargers
must be smart. And it's temperature sensitive. The good
news is that the typical cycle for an outboard (or car) will
discharge the battery a bit, then alternator catches up. But
if I were driving a whaler to Hawaii (yes, I'm being a bit
silly here), I'd want a smarter charging system.

In the normal operating mode, an outboard (or car) is a
voltage source, not a current source. It supplies a voltage,
and, within reason and the capabilities of the alternator,
the current is what it is.


Moe posted 03-17-2004 11:27 PM ET (US)     Profile for Moe  Send Email to Moe     
Chuck, my friend... you must be loaded up on Irish whiskey and green beer tonight like I am! ROTFL! You got it backwards.

Go back and read my post again. I suggested a flooded cell battery for the trolling motor because of its higher amp-hours. The AGM battery should be used for the starting application, which is a more critical application.

Best regards,

Chuck Tribolet posted 03-17-2004 11:38 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
No green beer (a couple of Coors though), no Irish whisky,
but the key point is why different types? The purpose of
AGM is to keep the acid inside the box. It's beyond the
envelope for most, if not all, of us to need AGM.


Moe posted 03-18-2004 12:03 AM ET (US)     Profile for Moe  Send Email to Moe     
Chuck, the best thing to ever happen to Harleys was AGM batteries. They're a lot more resistant to shock and vibration. They aren't just for jet aircraft (which are actually pretty smooth). They'll hold up better in boats which take a pounding.

I know you think you've never been in a situation where an AGM battery would've benefitted you. But I've never been in a situation on the street, in 40 years of riding, where a motorcycle helmet would've benefitted me.

We have to look at the data and not our own ancedotinal evidence when analyzing these things.


dan firth posted 03-18-2004 09:15 AM ET (US)     Profile for dan firth  Send Email to dan firth     
I moved dual batteries into the console on my old 18 OR (Cetacea page 16) and went with the AGM for three reasons: 1)I didn't want to worry about keeping the fluid levels up. There's no room for me and the batteries inside the console and I didn't want to disconnect and pull them every time I had to check/fill the batteries. The AGMs are completely maintenance free. 2) I wanted to ensure that there was no corrosion source inside the console. 3)Much rougher ride for the batteries amidships than at the transom and we get some exciting seas on the Monterey Bay sometimes.

I used the two flooded batteries for trolling when flyfishing in fresh water. Those I would mount back in the transom using the tan battery boxes, then remove them when I went into the salt water.

Worked great for me.... Moving the weight forward offset the heavier 4 stroke motor that replaced the old Evinrude.


Chuck Tribolet posted 03-18-2004 10:51 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
OK, where's the evidence that a whaler needs an AGM battery?

Miltary jets use AGM batteries because they pull negative


jimh posted 07-17-2005 06:04 PM ET (US)     Profile for jimh  Send Email to jimh     
mfrymier posted 07-21-2005 02:48 PM ET (US)     Profile for mfrymier  Send Email to mfrymier     
One big benefit to many whaler users of the AGM type of battery is that they hold their charge for longer periods of inactivity significantly better than Flooded type batteries. I have optima blue-top AGM's in my boat for exactly this reason. Put your boat away for winter, fire it up in spring... (of course I live in California, where winter is more of a state of mind...)

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