Hello

I have a circuit with one small form P.S (Meanwell SP240-12) with a declared power of 240W,
and it calibrated to 14.5V.
it connects to a Load which takes max of 140W,
and in parallel, I have 2 SLA batteries (12V@9Ah) as a backup for the P.S (through 20A diodes).

the problem starts when I connect it all, somehow the P.S doesn't have enough power to keep everything working.
when I measured the input Voltage without connecting it to the load or batteries it shows me 14.5V - thats OK
but when I connect the Load or the batteries it drops to 12.5V and its not enough.
the batteries are fully charged so it not them.
I already tried to change the diodes it didn't help.

when I replace the P.S to laboratory P.S it works fine, so it seems the problem is in the P.S.
my question is why the small form P.S doesn't keep the load as he should?

 

hi T,
Have you measured the actual current output of the SP240, when its reading 12.5V [ on load].
It may be going into auto shut down with the initial surges On current. [Overload state]
E

 

yes I did, when only the load connected the measure shows in point 1 (input current) ~9A
and in point 2 (output current) ~ 8A
when I connected the load with the batteries point 1 (input current) ~11A and in point 2 (output current) ~ 12A
which means the batteries also supplying current that doesn't need to.

The power calculation gives 16.5A (240W/14.5V) the Load takes up to 12A so I have a "free" 4A for the batteries
how can I make sure the batteries takes only 4A?

 

calibrated to 14.5V.

The PS adjustment range is specified as 10-14V, so it may well not hold 14.5V. Besides, as per the above datasheet, over-voltage protection kicks in at 14.7V (no tolerance specified), so 14.5V is perilously close to that and any transient may trigger the protection mode.

 

hi T,
As I understand your project, the batteries have to supply the 140W to the load when the PSU is not connected.??
If you add charge current limiters to the battery inputs, it will you have to some method of by passing the limiters when running on batteries.

This could be done using a relay/contact that by passes the current limiter.
E

 

The PS adjustment range is specified as 10-14V, so it may well not hold 14.5V. Besides, as per the above datasheet, over-voltage protection kicks in at 14.7V (no tolerance specified), so 14.5V is perilously close to that and any transient may trigger the protection mode.

I tried with 14V and it doesn't help so I just rotate the limiter to the maximum.

hi T,
As I understand your project, the batteries have to supply the 140W to the load when the PSU is not connected.??
E

yes exactly and when it connected the P.S need to keep the batteries charged at 13.8V.
that's why a relay won't help, the batteries won't charge.

if I change the P.S to Sp240-15 model
how can I know it will work?


Thank you

 

that's why a relay won't help, the batteries won't charge.

Hi,
The relay is only used when bypassing any charge current limiting device to the batteries.
So why do you think it will not work.?
E

EDIT:

 

I will try it and report

Thanks everyone

 

hi,
A source of 12Vdc relays with a high current contact ratings can purchased from your local motor accessories shop, or the local car scrap yard.
E

BTW:
If you have 'lots' of say 5Amp diodes, you could drop the voltage going to the actual batteries, by connecting in series with the battery inputs, this will reduce the charge current, acting as ''current limiters'' [ still requires the relay by pass contact]