12VDC Backup supply

[Audioguru]

No. I have done a lot of stuff with rechargeable batteries from the MOD down. NI MH and NiCad batteries can be charged indefinitely at the 14 hour rate which is essentially charging at 10% of capacity. The 14Hr rate comes from the charge process only being 60% efficient.

Energizer says that their old Ni-Cad batteries are fine charging at the 14 hour rate continuously but Ni-MH must have the trickle charge reduced to 1/40th or their life is shortened. New (the last 2 years) Energizer Ni-MH cells are the new kind (like Eneloop) that come pre-charged and hold a charge for 1 year.

Have you ever charged Ni-Cad and Ni-MH battery cells at the same time? Their thermal characteristics are opposites. The Ni-Cad gets cool but the Ni-MH gets warm and when fully charged the Ni-MH produces a terrific pressure of gas inside that damages it if overcharging is continued.

I haven't used Ni-Cads for about 20 years. They developed metal whiskers inside that shorted them and I needed to blast a whisker away with a high current then soon another one would short the cell. Ni-MH don't doo dat.

 

[partyanimallighting]

 

[(*steve*)]

That's better. Now as the resistor for a 200 hour charge rate.

 

[partyanimallighting]

Hi guys, found these online. 12VDC 1300mAh so they should hold a charge for a good while compared to the Energizer 9V 175mAh rechargeable. Opinions?

 

[(*steve*)]

Have you addressed the issue if the power required by your device when not in standby mode?

What is its peak power requirement and can that be supplied from your auxillary battery, and for how long?

 

[partyanimallighting]

Now, based on what I've learned here, if I use one of these batteries, I'll have to put a diode on the main battery + feed to prevent the backup battery from discharging into the line. I believe I'll have to use some type of resistor on this main battery + to prevent an overvoltage flow to the backup battery, so I believe a resistor will provide a trickle flow to charge the backup. I suppose I can solder the diode and resistor directly into the line and seal them up with heat shrink. In the event that there is overheating and a fire risk, will the heat shrink be sufficient protection? Should I wrap the diode and resistor in insulating paper also? What about the battery possibly overheating while the vehicle is running? Should the resistor prevent this?

 

[partyanimallighting]

 

[partyanimallighting]

The power required in standby mode is only 0.2mAh. I would think the 12VDC rechargeable at 1300mAh would be a good choice.

 

[(*steve*)]

Now you've removed the necessary diode.

Why?

And we still don't know that the unit will work when not in standby mode.

 

[partyanimallighting]

Here you go. So one diode prevents power flow from the backup in the main line, the other prevents overpowering the backup battery from the main battery and the resistor allows a trickle flow from the main battery to the backup for a slow charge. Am I correct? As I'm a novice, can you explain the calculations to get the values of the diodes and resistor?

 

[Audioguru]

The Huosuli battery is probably Chinese. Then it might be very poor quality and not produce its rated 1300mAh and it might use old fashioned chemicals then it will not hold a charge for more than a few weeks.

It has 10 cells that average 1.2V during a discharge and it is about 14V to 15V when fully charged that might not be possible with your "12V" source.

 

[(*steve*)]

Yeah, I agree with @Audioguru if you want to do this, stick to the 9V battery.

However, you seen to have completely ignored the issue if the GPS when not in standby mode. Surely the point of this battery is to provide power to the GPS to allow it to perform it's tracking function when the main battery is disconnected. Do you have any idea how much power this requires?

 

[Audioguru]

A new Energizer Ni-MH 9V battery is rated at 175mAh. It should not be trickle charged higher than 175/40= 4.4mA.
Its voltage is about 8.4V at half a charge and your 12V source might be 13.2V then the diode drops the source 0.7V to 12.5V and the resistor value for 4.4mA is (12.5V - 8.4V)/4.4mA= 932 ohms. Use 1k ohms. The power rating of the resistor should be 12.5V squared/1k ohms= 0.16W, a 1/4W resistor is fine even when the battery is completely dead at 0V.

You know what? The "9V" battery is fully charged at about 9.8V so the resistor value can be as low as (12.5 - 9.8V)/4.4mA= 614 ohms. Use 680 ohms. A 1/4W resistor will get hot so use a 1/2W resistor.

 

[(*steve*)]

And you can refer to the formula I gave previously that is correct for this instance. My recommendation was for a lower charge rate. (I went for 200 hour, @Audioguru went for about a 30 hour)

 

[partyanimallighting]

OK. So we're sticking with the Energizer 9V rechargeable, which I can easily hide inside a door post. Use the diagram above and use two 1N4001 diodes and a 680Ω 1/2W resistor. Correct?

 

[(*steve*)]

That's fine, as long as you don't need the GPS to actually operate.

I'd use a much higher value resistor.

 

[partyanimallighting]

A new Energizer Ni-MH 9V battery is rated at 175mAh. It should not be trickle charged higher than 175/40= 4.4mA.
Its voltage is about 8.4V at half a charge and your 12V source might be 13.2V then the diode drops the source 0.7V to 12.5V and the resistor value for 4.4mA is (12.5V - 8.4V)/4.4mA= 932 ohms. Use 1k ohms. The power rating of the resistor should be 12.5V squared/1k ohms= 0.16W, a 1/4W resistor is fine even when the battery is completely dead at 0V.

You know what? The "9V" battery is fully charged at about 9.8V so the resistor value can be as low as (12.5 - 9.8V)/4.4mA= 614 ohms. Use 680 ohms. A 1/4W resistor will get hot so use a 1/2W resistor.

Hi Auidoguru, I'm trying to get the hang of the calculation. 175/40 = 4.375 ~ 4.4mA. Why is 40 used as the divider? I am just trying to understand the calculation so I'm assuming there's some sort of set formula.

 

[partyanimallighting]

Hi Steve, I don't understand what you mean by not needing the GPS to operate. Will using the lower value cause a malfunction or not cause the GPS to function at all?

 

[(*steve*)]

You have based everything on the GPS being in standby. The 9V battery will cope with that.

But what happens when it comes out of standby to report (via GSM?) The location of your vehicle?

This has nothing to do with the resistor, and a lot to do with the battery.

 

[partyanimallighting]

Right. Understood. The current drain on the battery when the GPS is actually called upon to function to transmit data. Wouldn't a larger battery with the ability to handle a greater load be the solution to this?