How to disable charger circuit on APC UPS

Hello, I have this question posted in another group as well but I
figured I'd give you folks a shot at it.
I have two APC rackmount ups units. One is a 2200XL model which has a
larger internal charger to support external battery packs. The other
unit is a 2200 model that does not support external battery pack and
thusly has a smaller charger. I want to disable the charger in the
second unit while retaining all of the normal functionality of the UPS.
I want to do this in order to have them share a large deep cycle battery
pack where only the other unit will charge it.
I have taken a few photos of the main board of the ups. I am curious if
anyone is familiar with it or can recognize what I can do to disable its
charger. Perhaps even removing a simple fuse would do the trick, I
dunno. I can do the work, just not sure how to proceed. I do not have
a schematic for this.
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My first thought would be to stick a suitable diode* in series with the wire going from the external battery pack to smaller UPS. Bingo - no charging current from the smaller unit.
Or a suitable FET circuit with a much lower voltage drop than a diode will give - either as a simple two terminal diode replacement, or a switch operated by sensing some parameter that indicates the other charger is operating, or mains supply is present, etc.
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Perhaps I devised a problem where none exists. Let me step back and ask it this way; Is there anything wrong with running multiple chargers on a single battery pack? In my case, each charger will be onboard a 120V UPS system, and each system will be on opposite halves of a split 240 feed.
The charging power for the batteries is obviously DC, but I wonder if there is any risk of some sort of "bad AC thing" happening with dual chargers where each charger is fed from a split 240 leg to shared neutral.
Pal>> Hello, I have this question posted in another group as well but I
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There is a problem of running some internal battery UPS designs with exteral (larger) battery packs - when they weren't designed to do so. eg the manufacturer may have relied on the battery discharge characteristic to keep certain components within their safe operating region. A device that won't get hot enough to fail in the 15 mins runtime that the standard batteries can provide could get heated to failure in the 30 mins or more that an external pack can provide.
Of course such a unit may also have protective trips - but these may be non-resetable.
Some charger designs could give you problems - eg if they monitor the charging pattern of the battery and compare it to a standard pattern. Or monitor battery temperature and use that as a control parameter. A battery with a different charging pattern, with or without a second charger, can be interpreted as a faulty battery. This could lead to the charger disabling itself and alarms going off..
So you need to check those factors and more before proceeding further - or take the risk that all will be well.
-- Sue
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