Charging one battery bank directly from another battery bank

[force9BOAT]

Hello,

My RE system includes a 24V battery bank charged by solar PV.  I also have a battery powered lawnmower with internal 24V sealed lead-acid batteries.  I'd like to charge the lawnmower batteries off my RE system rather than the AC adaptor the mower shipped with.  But I'm wondering if there are concerns regarding charging one set of batteries from another (larger) set of batteries.  After heavy use of the mower there might be a significant voltage difference between the RE battery bank and the mower batteries.  I'm worried I might cause a damaging boost charge rather than a trickle charge when I connect the charging cable.  Would it be a good idea to add a resister (maybe one or two ohm and maybe also a diode to prevent backflow) in the charging cable?

Thanks all,

Rob

[drdongle]

Charging one battery bank from another of the same voltage is not as simple at it may seem. To properly charge, the voltage into the battery must be higher ( 27.6 V or so) than the nominal battery voltage, (24 V in this case). So what you will need is a DC to DC converter to "boost" the main battery voltage high enough to charge the mower.

Another possibility ( and simpler way)it to use the solar panels to charge the mower as they will have a high enough output(probably about 34 V open circuit) this is how it charges the main bank.

[stephent]

Personally I would use the original charger (charge matches batteries well--hopefully) with an inverter and accept the losses instead of cooking the batteries (IF the original charger will even work with your inverter--some MSW type inverters don't play well with certain chargers.)

Manual controlling the setup you propose can lead to grief quick. But YMMV and it's your batteries as well.

[richhagen]

Since your lawnmower has a sealed lead acid battery bank, you will not want to exceed a certain current level in order to avoid excessive gassing.  Excessive gassing would not convert back to water quick enough, and could allow pressure to build inside the battery.  Most sealed batteries I have seen have sealed vents that will open if the internal pressure becomes to great.  The batteries usually contain a catalyst to aid in the recombination of hydrogen and oxygen, which keeps the battery from drying out.  The opening of these vents lead to escape of hydrogen and oxygen gas and ultimately dry out the battery, ruining it.  

The bottom line is don't exceed the battery manufactureres limitations of voltage and amperage rate for charging the sealed batteries.  If charging them from a higher voltage bank, you may need to limit the current via resistance or another method to stay within the recommended charging specifications.  Rich

[willib]

i agree with rich dont exceed the charging current on the SLA batteries

why not charge the lawnmower batts with the windmill? both are 24V ..

or in other words just switch your input to the re system over to the lawnmower batts when they need charging

how is the windmill doing?

[force9BOAT]

Thanks for the comments about limiting charging current.  That is exactly what I was concerned about.  My batteries are connected to my PV as well so in effect the lawnmower battery will be getting charged from the PV cells.  I was only concerned about the initial connection if there was a significant voltage difference between the RE battery bank and mower battery.  I think I'll add a small resister to insure limited current flow.  The AC adaptor puts out 1 amp so I'll add resistance to stay close to that.

My wind turbine with new blades has been up for a couple months now.  And I had one tree that was directly behind the turbine cut down.  I needed to do that anyway for other reasons (couldn't justify cutting the tree solely for the turbine).  But as yet I have not had wind adequate to reach cut-in speed.  So I just enjoy watching the blades turn at about ten RPM most of the time.  It looks pretty.  I'm waiting for the first big winter storm.  

Eventually the wind turbine will by moved to my vacation home on the beach where the wind blows hard all the time and without trees in the way.  For now its just a fun toy.  The PV does all the work.  I'll post some new photos when I get time.

[finnsawyer]

You could use a rheostat (remember those) in series with an ammeter and the batteries to control the current.  You start with the rheostat resistance high and reduce it as the battery charges to keep the current within the allowed value.  The advantage is that you minimize power loss in the series resistor.  The disadvantage is that you need to keep adjusting the resistance as the battery charges until its voltage gets high enough to keep the charging current at a safe value with zero resistance.

[Ungrounded Lightning Rod]

Or use a big lamp (like a headlight, or several in parallel) for the resistor.

Lamps have a temperature-variable resistance, high when hot, low when not.  Pick the right bulbs and you don't have to crank knobs (though it will take a bit longer since the resistance is non-trivial even when cold.)

Even better would be a heavy DC motor driving something with enough friction to stall it when its input voltage gets down to a volt or so.  Spinning motors have back-voltage while stalled ones have only their winding resistance - which is very low.  Be sure it can handle the charging current when stalled.

[bj]

    I wouldn't do that.  Here's why.  I used an old car battery to

turn over a smallblock I had just finished.  Battery was old, and

should have been thrown out.  Anyway, it cranked a few rounds, (I was

just setting #1 Cyl, and no sparkplugs) and gave up.  So of course

I grabbed the battery charger, and hooked it up.  Gave it 10 mins.

Hit the starter again, and was lucky enough, or smart enough, (doubtful)

to be wearing goggles.  The charger clamps arced a bit, and the battery

had a dead cell.  Hence lots of hydrogen, and oxygen.  The  battery lids

are yet to be found, and all four sides split.  I was close to water,

and goggles protected my eyes.  Clothes did not survive washing.  A

couple of small puncture wounds.  I never was pretty anyway.

    After all this, BE CAREFUL with batteries.  Don't overcharge, and

don't use what you know is garbage.

    bj