Backup DC supply - design or purchase - electronics genius needed!

I am trying to find - without success and thus now thinking of building - a device to switch to a backup 12v battery supply when the main (nominal) 12v DC supply drops below a certain voltage.

This low voltage condition occurs regularly but briefly due to another device cutting in, but is soon restored, however the temporary voltage drop is causing havoc with something else!

When in the non-swotched condition the main supply would be used also to recharge the 12 v battery.

A friend has suggested as an alternative I could fit a capacitor wth a diode to prevent discharge 'upstream' of the device that needs protecting but how on earth do I calculate the capacitance required?

 

Use a relay to switch between the two batteries. Connect the "Normally Open" contacts to the main battery; the "Normally Closed" contacts to the backup. Here's the circuit schematic that drives the coil: NE555 based Low Voltage Battery Cutoff Circuit

A battery isolator or battery combiner will take care of the charging problem.

 

The diode and capacitor is the easiest. Need to know how much current your device draws and how long the power dip occurs. Only if it is for high power or long outages do you really need a battery.

 

I havent explained myself very fully; it is actually an automotive application.

- the voltage drop is due to driving an engine starter motor. The piece of electronic kit doesnt like the voltage drop so cuts out. When the engine is started and alternator comes on line the voltage is restored but the data etc on the electronics has been lost

The current to power the electronics is about 1A maximum and of course its a nominal 12v.

I am informed that a capacitor to supply this current for say 5 seconds would be about 0.5F. I have also been told that if you switch on a supply to this when it is discharged, it will draw a large current for a short time, maybe up to 120A to charge up in a few milliseconds, so its not as simple as using a capacitor there needs to be some sort of current limiter too!


Final issue is that the electronics gives up the ghost at about 10v so the capacitor is going to be of no use once it has discharged a certain amount, i.e. we cant get the full capacity from it

All the above means little to me as I dont reallu understand electronics - electrics I can cope with!

 

1A is a lot of power for backup memory. What does the unit require total ?

Is it a three wire device.... 12v ignition, 12v memory hold, ground ?
If it's only a two wire device then keep it off until the vehicle is started.

You can't use a capacitor in your application because a cap can't hold that much of a charge. You can do it with a small lead acid battery. Maybe 12v 1-2 A

 

How about a small, sealed battery or motor cycle battery to power your electronics. Connect the battery to the car's electrical system with diodes to isolate it and prevent the back flow of current when starting the engine.

 

I guess most of the power is the screen (Its a glorified Satnav basically) as the tech specs say it draws 13W at maximum brightness and minimum op voltage of 10.2V I worked that out as 1.3A approx.

Turning it off prior to start is the same as it turning itself off during start! The problem being that it takes a good two or three minutes to re-boot and find itself and it can be time critical (I'm not trying to be mysterious but dont want to bore you with all the irrelevant detail, but will if you want)

The battery with just diodes could work I suppose, but when engine was running the SLA battery would be getting a constant 13.8V across it - not good surely? It needs a charging device with it or something?

 

Satnav...ok That's basically the equivalent of our GPS units here. Ours have a built in battery and the vehicle startup is not an issue.

Is yours wired with the key so that when you turn the key on it comes on ?

Give us some details....you won't bore us.

 

OK - I see it will be easier to explain the exact situation rather than simplify it - which is actually making it more complicated! The 'vehicle' is a sailboat. The device is a GPS Chartplotter. The plotter runs off battery when sailing but usually when coming in to a busy harbour or area with difficult navigation around rocks etc it is prudent to drop the sails and motor (no problem going in to wind etc and it gives better visibility) This is the last time that you need to lose your navigation ability for three minutes!

No -its wired into a separate circuit with a master switch, but once that switch is on its at the mercy of the vehicle voltage drop during engine start.

More modern boats have independent electrical systems so there is a deicated battery just for engine starting that isnt connected to anything else. To convert to that would be very expensive compared to what i hoped would be a simple off the shelf device or cheap electronic build!

 

Ah....ok. Makes perfect sense now.

Let me think on it. I'm out on the road right now. I started drawing a circuit using a small battery.

 

Whoa, whoa... first the important stuff. What kind of sailboat. Where does it spend most of it's life. Pictures wouldn't hurt and could give us inspiration. I'm fond of Beneteau's but my wife loves full keeled ketches.

I've seen that problem with undersized wiring and/or poor connections. Check all the connections as it takes only one bad one to drop the voltage under high load. Unhook each and polish/sand both down to shiny metal, apply some dielectric grease and reconnect.

It can also be a sign that your battery is nearing the end of it's life. It may show good voltage at rest but can't deliver under load. Most auto parts stores will load check it for free.

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And just to be a bung hole this was us coming out of Anegada earlier this month.

 

Pilot Dane's idea of a separate Sealed Lead Acid battery connect to the main power supply through a pair (really only need one but they are cheap) of blocking diodes is inexpensive and reliable. That is what I would do. The 13.8 volts from the generator is perfect as a SLA battery is rated for 13.5 volts when in a "floating" condition and the diodes will consume the extra three-tenths of a volt.

I did some electrical work on the Dorade back in the late 1960s.

 

The correct way to do this is to have "house" and "start" batteries with a marine-rated isolator between them to prevent the starting battery from being discharged.

Here's a sample.

 

In Skyflyer's last post he said that he was aware of how more modern boats have separate battery banks.

 

I saw that, but in the first post said he needed to switch to a backup battery. Installing a house battery system will cost about $100 for a battery, another $50 or so for an isolator, and $20 for wiring. That's the correct way to do it and about as cheap as it gets.

 

While I agree that a "house" battery system would be best, it is hardly the least expensive method. A 7 ah SLA is about $25. Add a couple of blocking diodes at less than a buck a piece and maybe another five bucks in wire and supplies. If the GPS unit is plug/jack connected you might need another couple of bucks for an additional plug/jack. Total cost forty dollars or less and MUCH easier than installing another battery along with charge divider and isolator.

 

For a short duration starting period.....a 7ah battery wouldn't even be needed. A 2ah would suffice. The switch is needed to stop the battery from being discharged when the main 12 v supply is shut off...... like during periods of non use or storage.

 

You're right, PJ, the 7 ah is far larger than necessary. I just used that as an example. Since most residential alarm systems use a 4 ah battery they are readily available, even in the homecenters. Prices for all of these sizes are still in the $16 to $25 range depending on where you buy.

Thanks for the drawing, I wish I could do those.

 

Kevin (hvactechfw) showed me. It's the paint program in almost every computer.

 

I've tried Paint several times and all I can do is draw really crappy crooked lines. Getting the captions correct was also difficult.

 

This is for a boat, not a school experiment. The wiring and switches should be rated for marine use.

Although the OP says the GPS is the only thing he'll run, that's never how it really works out. There are lights, stereo, fridge, and plenty of other systems that this rig will probably eventually be running. If any one of them gets connected there will be a fire hazard. And you know that one day the starting battery will be dead and this rig will be pressed into use as a booster.

 

The OP asked for a solution to a specific problem and one was presented to him.