[Boatanchors] Question on 24 volt

[Brian Clarke]

Hello Bill,

Just a slight refinement on Jim's advice:

Be careful about your choice of batteries.

You can get automotive batteries that are good for starting, lighting and 
ignition (SLI) - in cars - and are cheap. These are usually flooded 
lead-acid batteries. And they certainly have very low internal resistance, 
which is good for starting things that draw 40 A - but not good for things 
that draw 40 A continuously. For such service, you will need bigger than 
normal SLI service batteries, like deep-cycle batteries, which will not be 
as cheap as SLI service batteries. But like all flooded-cell batteries, you 
need to be careful of the gases given off during both charging and 
discharging; the gases come from the destruction of water and are in fact 
hydrogen and oxygen in exactly the right proportion to recombine 
explosively; this is why you need to top up the water. It is a silly myth 
that lead-acid batteries give off only hydrogen.

Be careful of using a fan around batteries. First, make sure they are 
brushless so there is no likelihood of sparking. Second, when batteries are 
being discharged vigorously, or when they are being charged after they are 
fully charged, the bubbling will carry off sulphuric acid, which your fan 
may carry to things you don't want sulphuric acid to eat. Better to have the 
right size batteries and a charger that cannot over-charge or cause gassing, 
except under highly-controlled conditions - equalisation.

Alternately, you could go for sealed lead acid batteries (SLABs), sometimes 
called maintenance-free batteries (MFBs) or gas valve regulated (GVR) 
batteries. These are the kind often used in golf carts and mobility 
perambulators for handicapped people. The beauty of these batteries is that 
if the cart or perambulator tips over, the electrolyte doesn't spill. But 
because the electrolyte is in gel form, the electrons don't travel so 
rapidly and consequently, the internal resistance of such batteries is quite 
a bit higher than in flooded lead-acid batteries. So, the batteries need to 
be bigger capacity than SLABs for the same continuous load.

Use plenty of petroleum jelly, eg, Vaseline, on the battery interconnections 
to prevent corrosion of the terminals; auto supply shops can also provide 
spray-on anti-corrosives.

I suggest you visit an alternate energy supply house, eg, one that supplies 
solar PV cells that work through an inverter to provide mains AC power at 
night, or possibly a place that supplies batteries for caravans. They will 
be able to advise you on the correct size and type of batteries for your 
load pattern.

Otherwise, what Jim advises is pretty good.

73 de Brian, VK2GCE.

On Saturday, March 30, 2013 5:01 PM, Jim Wiley said:


> One possible solution, and one that has other benefits as well:   Get a 
> 24-volt "Farm" automatic battery charger. rated at 10 amperes or so.  Get 
> either 2  each 12-volt deep-cycle marine or 4 each 6-volt golf cart 
> batteries (better).  he batteries are available at reasonable prices at 
> Costco or Wal-Mart.  Hook the batteries up in series, and presto, a 
> 24-volt system good for perhaps hundreds of amperes.  The batteries supply 
> the 40 amperes you need when running the gear, and the charger recharges 
> them overnight when you are not using the radios.  Advantages: lots of 
> amperes withouthaving to spend big money for current capacity that is used 
> only intermittently, immediate backup power in case of power outage, and 
> so on.
> That is what I use, anyway. Has worked fine for many years, the only 
> important thing to remember is to check the water level in the batteries 
> every month or so and refill them with distilled water as needed.  Put the 
> batteries where theyare well ventilated. I have a pair of almost silent 
> salvaged 12-volt computer fans (wired in series for the 24 volts) blowing 
> across the tops of the cells to disperse any hydrogen they might generate. 
> Theuse of a relatively small battery charger limits the amount of gas that 
> the batteries can create at any given time.
>
>
> - Jim, KL7CC