#140493  by rawill
 Mon May 14, 2018 2:50 pm
Out on my Rideon lawnmower and got to thinking.
Not a good idea to think too much I guess!! :-T

What are the reasons I could not connect my 12v solar array directly to my 24v batteries via a diode and fuse.
With out a data logger to check exactly, it seems to me that the voltage would rarely be above 27.5, and often around 26.0v.

Just adding more confusion to the discussion I guess.

Robin
 #140495  by rawill
 Mon May 14, 2018 4:12 pm
Neddy wrote:
Mon May 14, 2018 10:53 am
Robin, that device is a really just a voltage controlled switch, a VSR. It is either "On" and connecting uncontrolled solar panel output directly to the Starter battery, or it is "Off" and the battery isn't being held at Float voltage. The device is continually flicking back and forth from "Charging" to "Off", rather than steadily holding the Starter battery at its Float voltage, as desired.
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Neville.
I did read up on this unit for a while before I ordered it.
I guess the diagram with the unit connected to the solar panels confused me a bit, as did some of the wording.

I connected it to the 24v array, and set it to come on at 27.00 and off at 27.8.
Hence the clicking on and off quickly.
If I connected it to the 12v array and set it to come on at say 25.5 and off at 28.00, it would connect the array voltage to the 24v batteries. I assume this would keep them topped up at whatever voltage the 12v array could produce. And with a diode on the input side to stop reverse feed would this work?
 #140501  by Neddy
 Mon May 14, 2018 5:59 pm
What you want is a Float charger, designed to deliver exactly 27.4 volts. This will then maintain the starter battery at its recommended Float voltage. What you have set up is a system that doesn't switch on until the Starter battery falls to 27 volts (too low) and then will not switch off until it hits 27.8 volts (too high). A Float charger would hold it at exactly 27.4 volts and would not be switching on and off.

"If I connected it to the 12v array and set it to come on at say 25.5 and off at 28.00, it would connect the array voltage to the 24v batteries. I assume this would keep them topped up at whatever voltage the 12v array could produce".

Well, yes it would - but you don't want your Starter battery "topped up at whatever voltage the 12v array can produce."
You want it held at the Float voltage recommended by the battery manufacturer.

Neville.
 #140581  by myles
 Tue May 15, 2018 6:53 pm
A :?: humours :?: tale of attempting this when a 'smart' DC charging unit is in the mix :idea:

I have a REDARC BCDC1240D - dual input (DC+Solar) 40A in-vehicle dc battery charger - which chargers the 12V house batteries from the 24V starter batteries when the vehicle is running - it also includes an MPPT controller for solar, but this is not related to this 'story'.

The trigger for the BCDC1240D to charge from the 24V starter batteries is a voltage above the typical resting voltage of the batteries (but below the batteries float charge voltage).

Some might see the issue here?

Using a small charging unit to trickle charge the 24V start batteries from the 12V house batteries all looked good, however, as the charger raised the voltage of the start batteries towards the 27V'ish float voltage, the REDARC charger detected the higher voltage and, thinking the vehicle was now running, kicked in to charge the 12V house batteries from the 24V start batteries. So the charger was happily charging the 24V start batteries from the 12V house batteries and the REDARC was happily charging the 12V house batteries from the 24V start batteries. Around and around it goes...

The REDARC does have a feature to only activate on ignition/switch that would solve the issue...
 #140595  by rawill
 Wed May 16, 2018 8:19 am
On the plus side, everything must be full, and the system must be robust, no fuses blew!

So Neddy, how about this unit, I fitted it 2 days ago after you told me about my not fit for purpose unit!
https://www.aliexpress.com/item/LTC3780 ... 4c4dDz5msS
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