charging gell cells from alt

[pete36330]

on my mc9,,I've got 2 8d starting batts,,my house bank is 8 6v gell cells ,2x4 for 24 volts,,while driving can I charge the gell cells directly from the alt ?  do I need to worry about the charging voltage from the alt to the house bank?do I need a seperate voltage regulator for the house bank,,thanks

[bevans6]

There are different types of devices to do this, here is one that purports to be designed for exactly your application:  http://www.yandina.com/mixedbattery.htm

The reading I have done on AGM and Gel batteries (I use AGM and some Gel in my race cars, so at one point I did a lot of research on them) is that they can take a lot of charging current but they don't like high voltage.  Some "automotive" charging systems run at 14.5 volts or higher (for 6 cell batteries) and that is generally considered pretty high for a AGM or Gel battery.  My bus, on the other had, is regulated at 27.8 or so max, or 13.9 volts for 6 cells, and everything I've read suggest that is fine for any battery.

I don't have a clue about the impact of different types of batteries with different internal resistances affecting charge rates, though, and I suspect there is a story to be told there.  Old wives tales about that I have in spades, actual information - nada   

Brian

[Sean]

I would not bridge Gels directly to the chassis batteries and alternator.  As Brian mentions, there are devices to do this safely.

AGM's, however, can be charged on the same profile (voltage settings) as flooded batteries   In fact, AGMs will accept much higher charge rates (amperage) than flooded batteries, one of their key advantages when charging from generator-driven chargers.

-Sean
http://OurOdyssey.BlogSpot.com

[Bill B /bus]

Gel cells are voltage sensitive. Haven't worked with them so I am not familiar with the charging specifications.
AGM charging voltage is higher than the wet cell. Float voltage is also higher. Printed on the battery 14.5 to 14.8 for charging. Float is 13.6 to 13.8. I was concerned with the higher voltage so I called distributor tech support. I was directed to the data sheet for the batteries and yep the printing on the battery  was correct.
Sealed batteries house and starting. That means no more watering for me!

Bill

[ccbmster]

I will probably find out that what I am doing is all wrong....but, I have a switch rigged up that I can throw when running that allows charge to float from my bus batteries to my house batteries.  Seems to work just fine for me.

[pete36330]

can I adjust the alt to put out 28.2 volts. so I won't overcharge the gell cells and still have high enough voltage to keep the start batts charged? I'm planning on connecting the house and bus batts together with a solinoid and 8ga wire ..so while I'm running I can switch the house batts to charge ..and be able to diconnect them when not running the eng..need advice ..I don;t want to burn anything up ,,

[Sean]

can I adjust the alt to put out 28.2 volts. so I won't overcharge the gell cells and still have high enough voltage to keep the start batts charged?

Again, No.  You can not parallel Gel and flooded batteries without ruining one, the other, or possibly both.  You either need to change your house batteries to flooded or AGM, or start batteries to gel, or else you need to use a charge controller made for the purpose.

I'm planning on connecting the house and bus batts together with a solinoid and 8ga wire

If your MC9 has the Delco 50DN alternator, #8 is way too small.  You need at least 2/0 to connect the house batteries to the chassis system.  The 50DN can put out 270 amps, and anywhere from 220-250 of those amps can flow to the house system; you need to wire for at least that amount.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

The way that I think of it, and it's probably not completely accurate, is that a battery's state of charge dictates how much current it will draw when charging.  If it has been drawn down to say 11.5 volts, then it will flow a lot of current when charging from a 14 volt source, while if it reaches a float voltage of 13.6 volts it will draw far less.  That is why you have to plan for the maximum current that the charging source can deliver.  If you happen to have a 30 amp smart charger, then the maximum current it could deliver at any state of charge is the 30 amps, so you could be fine with 8ga or even 10ga wire.  If your total maximum current draw to loads (like lights, vent fans, that sort of thing) is 30 amps, you can use the 8ga or 10 ga to distribute to loads as well - presuming your length of cable doesn't produce large voltage drops.  But that ability of the bus alternator to produce such large amounts of current is why you need to assume that at some point you'll be charging those house batteries at a high rate of charge and do the installation accordingly.  My favourite thing to do is  to leave a little inverter on for a few days with the bus unplugged, a week later I come out and the inverter idle current has drawn my batteries way down to auto-shutoff voltage.  If I start to charge from my 30 amp converter, no problem, but if I was cross-connected to the bus alternator, those batteries at 11.5 volts would get a monster charge!

Brian