Battery backup for car audio system preset memory? - Page 1
Forum section: electronics / projects

author: DTJ
author_rank: Super Contributor
timestamp: September 18, 2015, 07:07:17 am
content: I've got a 12V radio in a 24V commercial vehicle with a battery isolator, every night when the isolator is turned off the radio loses its preset memory settings.The isolator needs to be operated due to current draw from a variety of pneumatic controllers, 24/12V converters etc.I don't want to run a small 24/12V set up just for the memory backup on the radio.I was thinking of adding either a primary battery with a couple of diodes or a NiCd battery with a couple of diodes and a charge limit resistor. I imagine the battery voltage probably needs to be upward of 7.5 volts.I don't have access the the vehicle at the moment and wanted to do some thinking prior doing something about it next week. If it matters its got a bottom of the range Axis brand stereo fitted.Has anyone done anything similar or know what the *typical* current draw is for the memory back up?Cheers.

author: Delta
author_rank: Super Contributor
timestamp: September 18, 2015, 07:10:46 am
content: Just stick the smallest 12v SLA you can get in there somewhere...

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 18, 2015, 07:24:51 am
content: As @Delta said, pretty much the smallest battery you can find will probably do it.Remember that memory-backup batteries in computers, cameras, etc. are tiny "coin" cells that last for years.Of course, those are wired INTERNALLY directly to the memory chip.If you only have access to the EXTERNAL power input, that is a different problem.You must remember to ALWAYS turn OFF your radio to prevent depleting your backup battery when the main power disappears.What you are calling an "isolator" seems to be actually a converter of unknown design that converts 24V down to 12V.And it is not clear WHY it must be turned off if there is almost zero current being drawn by the radio while off.Instead of using a battery, you could simply use a crude Zener "shunt-regulator" to take the vehicle 24Vand knock it down to 12V for the radio standby current.Normally, we don't use shunt regulators like this because they aren't that good, and waste power.However, in this case, where you have a huge source (the 24V battery(s) and a microscopic drain (the radio standby current),a Zener shunt regulator circuit would work.Does the radio not have a separate input wire for standby current? All the vehicle radios I've seen have separate power inputs.

author: DTJ
author_rank: Super Contributor
timestamp: September 18, 2015, 08:48:19 am
content: That could be the best way, ~1.2AH for around $20. I would put a diode upstream of it to prevent backflow into the rest of the system. I wonder if never charging the battery above 12.5 or 13V would have an adverse effect on its life. The isolator is an isolator. It's a large mechanical switch used to totally isolate the batteries from the vehicle. They are common on trucks and commercial vehicles. I think you guys in the US refer to them "Cole Hersee" switches after one of the manufacturers.I don't want anything connected to the main batteries when the isolator is operated (open).The radio has a separate memory backup wire.I was mainly interested in finding out what the typical current draw for the mem backup might be.

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 02:35:46 pm
content: Memory backup will typically be under 5mA, as that is a design criterion put out by the manufacturers as accessory off key load to ensure the vehicle will still start after being unused for a month ( think airline long term parking). Thus you need a small SLA battery ( 1.2Ah will be fine) and a dedicated float charger driven off the 24V main supply when the vehicle is powered on. You can easily do this with a LM317T set to a 14V4 output, a diode after it, a series diode on the input ( reverse battery protection) and a resistor in series with the input to limit current to around 0.5A into the battery when flat. Resistor will be roughly 27R 10W wire wound, in a well ventilated metal box, so it can dissipate the heat safely without burning surrounding materials. LM317 on a small heatsink ( use an insulating kit to mount it to the one wall of the box), and protect with a 5A automotive blade fuse in a holder ( should be a 2A fuse, but they are hard to get, just make sure you use a non fake blade fuse, there are some where they are all the same thick 40A stamping with different colour housings with false ratings printed on them) for extra protection.

author: DTJ
author_rank: Super Contributor
timestamp: September 21, 2015, 03:26:08 am
content: Ok, 5mA sounds reasonable.I only really need it to hold up over a long weekend, 1200/5 = 240 = 10 days. Say 5 days in reality! I like the '317 idea, nice & simple.Thanks for the idea.

author: Magicmushroom666
author_rank: Contributor
timestamp: September 21, 2015, 06:48:04 am
content: When testing a car stereo myself quite some time ago, i found that the memory connection, was also the main draw of current when the stereo was working, the ignition cable was only used to wake up the unit etc. That may mean you need to put a relay to bypass the battery when the isolator is closed. Worth testing your stereo as things may be different in a modern unit.

author: DTJ
author_rank: Super Contributor
timestamp: September 21, 2015, 06:56:04 am
content: Cheers Magic - that could be a 'gotcha' - I'll check that out for sure.Thanks

author: cpuerror
author_rank: Regular Contributor
timestamp: October 10, 2015, 03:35:03 pm
content: Amazing that in this day of age car stereos still lose all settings when power is removed. The alternative to a battery is to get a marine radio which is designed to have the power cut off.

author: promacjoe
author_rank: Regular Contributor
timestamp: October 10, 2015, 05:31:26 pm
content: Normally, any radio that has a volatile memory, needs two power wires to operate. One power wire connected to the key switch and a second power wire connected to the battery, for the memory. these two power wires are probably connected together. There are two ways you can fix this problem.1: add a diode and a second battery/large capacitor to the constant power wire.2: run a second wire to the 24V battery, connecting it between the to 12V batteries. on many commercial trucks, this wire is already provided for you for this reason. check your owner's manual.Note it is not recommended to power the radio for it's normal operation using this method. it can produce an unequal charge load on the batteries and possibly damage one of the batteries. a draw of 25ma or less will not harm the batteries.Normally, at least in the US, truck manufacturers, Make accommodations for standard 12V radios, to be used in their trucks. This includes providing a memory wire for the radio. Again check your owners manual.