SLA charging in a car? - Page 1
Forum section: electronics / general

author: DTJ
author_rank: Super Contributor
timestamp: May 11, 2018, 12:33:10 pm
content: I've been trying to sort out how to charge a small SLA battery (<10Ah) in a car.I've looked everywhere for an off the shelf system with a cigarette lighter plug one end and a couple of terminals at the other.I think all it needs to do is limit the charge current to say 1 amp.The bulky alternative is to grab a can style 150W 12V to 240V inverter and plug a small SLA charger into that.I figure someone must make something like this but I can't find it on eBay, Amazon etc. Any ideas?

author: richard.cs
author_rank: Super Contributor
timestamp: May 11, 2018, 01:48:36 pm
content: The very manual approach is to connect the battery to the cigarette lighter plug via a resistor or even a lightbulb when the engine is running and disconnect it before stopping the engine. If you don't want the manual intervention then there needs to be some switching logic there, if you want to charge without the engine running then you also need to increase the voltage.I've not seen anything commercial for charging SLAs in a car, with the exception of some of the halogen spotlights but they have 6 V batteries.There's a pretty basic charing circuit here that works:http://ludens.cl/Electron/12Vcharger/12Vcharger.htmlbut the efficiency is low, probably around 50%. A quick google for "12V input sla charger" came up with this:https://www.powerstream.com/pst-pb1108.htmbut it's fairly pricey and intended to charge bigger batteries than you have.

author: bd139
author_rank: Super Contributor
timestamp: May 11, 2018, 02:27:09 pm
content: I’d use a cheap Chinese “150W boost converter” and an LM317. There are lots of circuits out there for this.Edit: there’s also a canned SLA charger IC but I can’t remember what the identifier is.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 11, 2018, 03:13:29 pm
content: Quote from: bd139 on May 11, 2018, 02:27:09 pmThere are many, but one that would be near ideal for this use case is the LT1513.

author: Ian.M
author_rank: Super Contributor
timestamp: May 11, 2018, 03:39:12 pm
content: One EBAY CC CV Buck-Boost module set to the desired float voltage and max charging current, with on its output, a MOSFET based ideal diode to prevent the battery discharging into the module's voltage sense circuit and its input fed through a choke then shunted by a beefy 18V TVS diode and a lot of capacitance to keep the worst of the automotive supply transients from letting the Chinese smoke out.

author: bd139
author_rank: Super Contributor
timestamp: May 11, 2018, 04:45:02 pm
content: I like the smell of Chinese smokeThis is the IC I was thinking ofhttp://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slus186&fileType=pdf

author: controlfreak
author_rank: Newbie
timestamp: May 11, 2018, 05:28:14 pm
content: What worked for me a few years ago to charge a 24v SLA battery bank in my vehicle was to get a "900w" CC/CV boost converter that had potentiometers to adjust voltage and current. Obviously, I would never put 900w of power through one of those, but it seemed quite happy running around the 150w I wanted.Initially I played around with digital potentiometer ICs to control the parameters before realizing the existing pots were just outputting a 2-6v DC signal across their range. That made it trivial to use two 0-10v channels to reliably adjust the output.Consequently, I was able to do three-stage charging simply by changing two analog values to match what I needed for the stages. A side effect is that I could limit the input current to whatever max value I thought was reasonable for its connection to the vehicle (it moved between a few).I'm currently working on a new setup for my new vehicle, and will probably be going down a very similar route again to charge an aux 24v battery bank in a controlled manner.

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2018, 07:45:19 pm
content: If you can get hold of a signal that shows engine running then simply use a relay to connect the SLA battery to the main battery bus when engine is running. That will charge it with the alternator 14V4 peak voltage, and will charge it to around 80% in around a half hour of the engine running. Basically you are making a split battery charger, without using the normal method of having a modified alternator and 2 bridge rectifiers to provide 2 separate isolated voltage rails, with the control being done on the primary battery for voltage control.

author: controlfreak
author_rank: Newbie
timestamp: May 11, 2018, 08:05:36 pm
content: Quote from: SeanB on May 11, 2018, 07:45:19 pmThat can work, but be careful. If the SLA batteries are isolated from the main battery but charging off the alternator you can have issues with vehicles that control the alternator field voltage. The controller will adjust the output voltage of the alternator based on what it believes to be the charge state of the main battery. Odd things can happen, but I'd mostly be concerned about overcharging the SLA.For instance, my new vehicle has a "smart" alternator that's supposed to decrease charge times and increase longevity for the starting battery. During its equivalent of the bulk charge cycle it's outputting around 14.8-15.2v, which might be okay for cyclic use on a vented battery but is higher than you'd want on an SLA to avoid gassing. I actually brought this vehicle back to the dealer after I noticed how high the charge voltage was, but they assured me it's normal and working as it's supposed to.If you just want to isolate a battery and use a relay to control charging, it would likely be simpler to use a vented cell instead of an SLA. In either case, a low/high voltage cutoff relay could be useful.

author: G7PSK
author_rank: Super Contributor
timestamp: May 11, 2018, 09:06:04 pm
content: You can get in car chargers for mobility scooters off the shelf, they output 24 volts dont know if a dropper resistor would work or if the output can be turned down.https://www.ebay.co.uk/itm/HIGH-QUALITY-IN-CAR-24v-2a-LEAD-ACID-OR-GEL-MOBILITY-SCOOTER-BATTERY-CHARGER/142117334568?epid=558154902&hash=item2116da6e28:g:oMIAAOSw-dBTuXUf

author: controlfreak
author_rank: Newbie
timestamp: May 11, 2018, 09:39:18 pm
content: Quote from: G7PSK on May 11, 2018, 09:06:04 pmAlong those lines, a towed trailer battery maintainer could work too. I think Optimate makes one that does 12v SLA charging at up to two amps from a 12v DC input.