24v to 12v Conversion

[jpfieber]

I have a 24v trolling motor on my 12'er, and will probably add a few 12v accessories (lights, radio, etc). I already have a 2-bank charger for the two 12v batteries I'll need to have wired in series. I don't have an electric starter to worry about, and I don't want to add a separate battery and charger for the 12v accessories. I understand that to get the 12v for my accessories, I could just wire that circuit to one battery, but then the batteries will be at different levels of charge, which I'm sure has drawbacks. One way around this would be to use a "DC/DC Converter Regulator 24V Step Down to 12V" ($20 for 20A on Amazon). This would give a 12v feed that draws evenly from both batteries. It seems to me this would be a better option, even with a 10% loss in the conversion.

Let's assume my accessories use 10ah (random number that's simple to use). If I connect them directly to one battery, that battery will eventually have 10ah less than the other, and my trolling motor run time will be limited by that battery which reduces run time by 10a. If I connect them via a converter, I would think that each battery would loose 5.5ah (10% loss figured in), so my trolling motor run time would only be reduced by 5.5ah. Would this actually be the case, or would each battery be reduced by 11ah, hurting more than helping?

 

[BigTerp]

I cannot answer your question specifically, but I run a 24V trolling motor using 2 group 27 batteries. The batteries are NOT run in series. This leaves each 12V battery still capable of running 12V accessories. I have the 24V trolling motor hooked up to the + of one battery and the - of the other. Then a jumper wire connected to the - of the first battery and the + of the second. This creates a 24V circuit for the trolling motor only. I have various 12V accessories wired to each battery. My one battery has motor starter, led nav, anchor and interior lights, stereo, sonar, 12V socket and bilge pump. My second battery has second bilge pump, LED and HID driving lights and spare 12V socket. I know this isn't the best scenario for battery health, but I have no choice if I want to run a 24V trolling motor since I have no room for a third battery on my boat. I typically fish 6 or so hours at a time and have my trolling motor running 90% of the time. No problems with this setup in the year plus I've been running it like this. I do keep a portable jump starter on my boat just in case I happen to run the starting battery dead. So far, I haven't had to use it and the motor fires right up after a day of fishing.

 

[Stan the man]

Sounds like you do have them in series,.. you just have them tapped for 12volts..
And if I were worried about draining one battery more than the other, I'd put in a three way switch so I could select which battery I wanted to use at any given time..

 

[.Mike]

I'm not an EE, but I do understand the concepts well enough to stick my hand in a tube amp running at 400+ volts, and to design fairly complex (analog) circuits.

To the original poster, yes, a DC-DC converter is one safe way to go. Who cares about minor efficiency losses when the end result is protecting $200+ worth of batteries and the boat in which they sit. 90% efficiency is pretty good.

Just to clarify a bit on your numbers. If you were drawing 10Ah, the batteries would be mismatched by 10A of capacity for every hour of use. After 4 hours, there would be a 40A capacity difference (and probably a decent voltage difference, too).

A typical 12V battery is made up of 6 individual cells in series. Once you connect the two batteries together, they can be thought of as one battery consisting of 12 individual cells in series. Treat them as one, and they will basically self-regulate, just like the 6 cells in a 12V battery. Your DC-DC converter will draw current evenly enough for it to not be a concern.

The information on why it is bad to run two batteries in series while drawing current from each independently is out there, and easily understood. When you draw current from one battery that is also sourcing current as part of a series, you lower the voltage of that battery. When you lower the voltage, the other battery sources more current to compensate for the weaker battery. The stronger battery could attempt to reverse charge the battery with the lower voltage, which is not good at all.

 

[Stan the man]

Mike,.. You said tube amp.. Might you be, a fellow Amateur Radio enthusiast??

 

[earl60446]

Only $20 for the converter or you could just source some accessories of each battery according to current draw and use.
Either way will work ok.
Tim

 

[.Mike]

Mike,.. You said tube amp.. Might you be, a fellow Amateur Radio enthusiast??

Nope, although it is something I have always wanted to do. Getting a license and some gear is in my future.

Playing guitar has been my main hobby since about 1990. This [link] is where I post some of my projects, although I haven't posted anything I've completed in the last couple years. These are my favorites. My current project is a lap steel with a black limba body with brown binding, and a rosewood fretboard with maple inlaid frets and mother of pearl markers. I'm in finishing now, and have about 25 coats of Tru-oil applied.

Sorry to be off topic.