Charging two different battery banks simultaneously

Boat Systems
1

Al,

I looked into the docs because I have the same challenge here. The echo-charge is a voltage-follower, according to the manual. This means that the chargecontroller that supplies the charge, decides on the voltage that is supplied to the main battery based on the state of the main battery AND, through the echo-charge, supplies this voltage to the other battery as well. So the voltage supplied to both banks is always the same, which is far from ideal with two totally different battery banks.

I have a Xantrex Pathmaker (now not in use) which does basically the same as the Echo charge: connect both banks when the charge voltage supplied to one of the two banks is above a certain set point.

The advantage of the Pathmaker is that you can set the connect, disconnect and high voltage disconnect points yourself. So if you connect the alternator to the AGM house bank, which can be charged up to 14.9v, you can set the Pathmaker to disconnect the wet cell starter battery at 14.6v already. In this way you don’t overcharge the wet cell.

See the charge characteristics of an AGM battery here:
AGM 4 step charging.jpg
On the other hand, the much higher current (amps) that the AGM’s can accept, will not be accepted by the small wet cell starter battery. Voltage and temp will rise quickly, perhaps disconnecting the charge source prematurely from the starter battery.

Another advantage of the Pathmaker is the high current connection; you can start your engine through a Pathmaker connection.

For you info I uploaded the manuals of the Echo-charge and Pathmaker here:
Xantrex Echocharge.pdf (618 KB)
Pathmaker documentation.pdf (1.07 MB)
NOTE: I have no interests in Xantrex.



— In FreedomOwnersGroup@yahoogroups.com, “Al Lorman” <ajl@…> wrote:

Alan:

I’ve been considering installing an Echo Charge to control charging from an
AGM house battery to a wet cell starting battery. The Echo Charge manual
doesn’t directly address this point. Do you have yours set up this way?

Al Lorman

That’s the beauty of the echocharge. The alternator only runs to the house
bank and does so directly (at least in my arrangement) not throught he
switch. The starter battery is charged at up to 15A/H from the house bank
controlled by the echocharge whenever the voltage of the start battery is
below that of the house bank and at a rate dependent on the difference. I
don’t recall if it has a setting to allow to go from AGM house to Gel
Starter at lower voltage but from AGM to wet cell as in your case is no
issue.

Alan

I have a Freedom 15 (now Xantrex inverter charger with a second
independently controlled charge output for the starter battery. That
works fine when on the German dockline or running the generator. But
perhaps that’s enough and is there no need to charge the starter
battery with the alternator.

The problem always is to charge two completely different battery banks
(one starter, 60Ah wet cell, four house, total 350Ah AGM)with one
alternator regulator.

— In FreedomOwnersGroup@yahoogroups.com
mailto:FreedomOwnersGroup%40yahoogroups.com , “Alan Kusinitz”
<akusinitz@> wrote:

Michel,

For what its worth I currently use a Xantrex echocharge so my Starter
battery gets charged from the house battery. My shorepower charger is
connected to both banks but this isn’t really necessary.

Alan

2

I installed a Pathmaker and have been very happy with it. Trying to solve this problem using diode based systems (which I used to have) doesn’t work due to the 0.6V drop across the diode as the battery on the “wrong” side of the diode doesn’t see the correct charging voltage.

– Geoff

3

George, the new generation of diode bridges have MOSFET’s without voltage drop.
But obviously there will still be the same charge curve for both battery banks as long as there is only one charge regulator. Neither the Echo-charge, the Pathmaker nor a diode bridge gives off two different charge curves.

4

He’s Starski I’m Hutch!

Combiner

I go the combiner route. I’ve installed this type on a couple boats with good success. I plan on using them on Marquesa.

George

5

Same here, all gel banks on previous and current sailboat. Flawless.

6

All these gadgets basically do the same thing, if I read the docs carefully. But the Combiner seems to be the most economic solution. I am first going to try to revive my Pathmaker and if it remains undead (there is a LED glowing, but it does not do what it’s supposed to do), I think I order a Combiner too.

Here’s an interesting quote from the Yandina Combiner website:

Q Won’t the smaller battery get overcharged if put in parallel with a large one?
A The whole process of charging batteries in parallel is naturally self regulating, naturally governed by terminal voltage and current flowing through the internal equivalent resistances. For simplification, each battery can be considered as a perfect battery that holds the charge combined with a series resistor representing the internal resistance, and a parallel resistance representing the self-discharge leakage current.

Lets say you have a charging source dumping out say 100 amps. If you have 4 batteries in parallel sharing this source the distribution of the charge current is governed by those internal voltages and internal resistances. From Ohm’s law, the charging current that flows into each battery is the voltage divided by the resistance. In this case the voltage is the charging voltage arriving at the terminals minus the actual internal voltage divided by the series resistance of the battery. A discharged battery is going to put a larger voltage differential over the internal resistance so more current will flow to it. A charged battery is going to put less voltage drop across the internal resistance so it will take less or none of the available charging current. A large plate battery which has a correspondingly lower internal resistance will demand more current because of this lower resistance. A small motorbike battery in parallel will have a high internal resistance and demand very little current.

All these demands for current result in a common voltage on the parallel circuit which is what the alternator or charger sees. None of the batteries will ever be “overcharged” as a result because the charging voltage is controlled. Even the smallest battery will only be seeing the same voltage so it is like the other batteries are not there. If you suddenly removed all the big ones and left just the tiny bike battery on the 100 amp alternator, the voltage would immediately rise because the amps trying to flow into the tiny battery have to pass through its internal resistance which is high. But instead of the voltage or current going to an excessively high value, the regulator in the alternator cuts the current back to a level that limits charging to a safe level.

Here’s the owner’s manual of the Yandina Combiner:
Yandina combiner100.pdf (29.7 KB)

7

Add my unqualified endorsement for a combiner (and the Yandina models in particular) instead of the other options. Other options can work but combiners are electrically simple, robust, very easy to install and, perhaps most importantly, they “fail safe” (they just simply stop charging all the batteries automatically, though the batteries will charge just fine with the normal, un-changed, standard alternator.) They truly are “set and forget.” No matter where the charge is coming from (shore charger, engine, wind, solar, whatever) and no matter which battery needs the charge, it all happens automatically.

Mine has run without a hiccup and without the slightest intervention from me for a decade. A friend has had a similar experience. If only all boat accessories were like this…

8

We will have batteries galore. Two house banks of 4 golf cart batteries each, two engine start batteries (one for the main engine and one for the Frankenmaker) one group 27 up in the forepeak for windlass and thruster and if we decide to install that little ICOM Ham radio of mine we will have a separate battery for that. Exactly how I’m going to wire all this up I have not decided yet and it give me a headache to think about it…

George