Hi,
I've built a 12V SLA battery charger using the LM5118. The input voltage can come from a 12V or 24V battery (automotive) (designed 5V to 40V)
I've added a simple circuit to monitor the output current and switch in/out an extra resistor in the RFB path to switch between an output voltage of 14.8V and 13.8V.
The initial design was done with Webench and fine tuned with the LM5118 spread sheet.
I used a DC programmable dummy load to do most of the initial tests.
Under resistive load conditions the design seems to work fine.
The moment I charge a battery I hit problems with current limit, particularly when charging a 12V battery from a 24V source.
For example, using the dummy load I can see the LM5118 current limit at say 3.5A (24V input)
If I connect to a battery to the LM5118 the load current is only about 1 to 1.5A. (I would like it to be up near the 3.5A as above.
If I take the same battery and connect it to my bench power supply at the same output voltage as the LM5118 and the battery suddenly wants to draw 5.5A as it is quite flat.
(The battery is 12V,17Ah, 5.1A initial current)
So why is the LM5118 limiting the current so much?
I seem to get the reverse when charging from 12V input. The current to the battery exceeds the current limit value found with the resistive dummy load.
I suspect that the battery, having a voltage of say 11-13V, is holding the LM5118 output voltage "up" and not allowing it to current limit correctly.
Does anybody have any suggestions or experience in this regard?
Is the LM5118 suitable for this application? Are Buck-boost controllers in general suited to this application?
Can I salvage this design or do I start over?
Thanks
Andre
