Need a review of a 24v SLA battery charger circuit. - Page 1
Forum section: electronics / projects

author: MarkS
author_rank: Unknown
timestamp: April 04, 2023, 11:00:18 pm
content: I'm working on a robotics project that will use 2 12v SLA batteries in a 24v configuration. While I can just buy a charger, where's the fun in that? I did a Google search and found something that seems like it may work, but I am wary of online circuits, especially ones that operate on mains voltage.The circuit I'm following can be foundhere, titled "24V Automatic Charger and Full charged indicator".My schematic is nearly a 1:1 copy, save for putting the mains voltages stuff off board (all panel mount), the use of a 115v to 28v transformer and two test points to make it easier to calibrate the two trim pots.Before I commit to this design and send the PCB to JLCPCB, is there anything amiss? Just realized the labeled "LIVE" and "NEUTRAL" blade connectors are incorrect as these are inputs from the transformer's secondary. Please ignore.

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2023, 11:21:49 pm
content: Well, the whole right side of the circuit just powering a green LED could be better used for, eg, switching to trickle/maintenance charging.What is urgently needed is a rectifier diode from output to input of the LM317.If you switch off the mains, the LM317 will otherwise be powered backwards from the SLA and will die.Apart from that: OK.

author: Whales
author_rank: Super Contributor
timestamp: April 04, 2023, 11:46:01 pm
content: Is R3 a standard 1/4W resistor or something bigger? 1K across most of 20V is about 0.4WEDIT: also please make sure to keep the mains side fuse

author: MarkS
author_rank: Unknown
timestamp: April 05, 2023, 01:09:32 am
content: Quote from: Benta on April 04, 2023, 11:21:49 pmThe circuit is no where near that advanced. It just shuts off the power to the battery when it is charged. At least, that's what is said in the link.Quote from: Benta on April 04, 2023, 11:21:49 pmSo, this:Doesn't seem correct, but I'm not too sure how to route this!Nevermind. A quick look at the datasheet showed me exactly what you mean, as well as recommended capacitor placements and values, of which this circuit is desperately lacking. I've got some work to do...Quote from: Whales on April 04, 2023, 11:46:01 pmGood catch! I have them set as 1/4 watt, but haven't ordered anything yet. Easy fix!Quote from: Whales on April 04, 2023, 11:46:01 pmMost definitely! The mains input, fuse, switch and transformer are all panel mount. The routing will be Live in->switch->fuse->transformer primary.

author: MarkS
author_rank: Unknown
timestamp: April 05, 2023, 01:08:31 pm
content: Thank you for the help, but I'm going to give up on this part of the project. I don't understand the circuit well enough and this can have tragic consequences given what it does. If it were anything other than a mains voltage powered device charging lead acid batteries in my home, I might be OK with not fully understanding it.

author: Martinn
author_rank: Frequent Contributor
timestamp: April 05, 2023, 01:15:26 pm
content: Quote from: MarkS on April 05, 2023, 01:09:32 amD4 cathode is not connected - look closely. Probably something off grid.With KiCad, I constantly hit N instead of M (for move) accidently, switching to a finer grid. If you are not very careful about that, your schematic degenerates in an arbitrary grid mess (happened to me more than once).Stick to 50 mil for schematic.

author: globoy
author_rank: Frequent Contributor
timestamp: April 05, 2023, 02:04:29 pm
content: This appears to be a simple float charger. These types of chargers are ubiquitous but not necessarily the best for long life of the battery. This charger doesn't temperature compensate so it's possible it will either under- or over-charge the batteries over time based on their temperature. It is probably ok if the battery remains at a constant temperature. I'd adjust the voltage at the battery terminals to match the sum of the float charge levels printed on the battery. I'd also add the diode that others have suggested.

author: vk3em
author_rank: Contributor
timestamp: May 03, 2023, 10:25:13 pm
content: You might be better off using a UC3906/BQ24450 or similar.