Gomadic Power Accessories for ALL !!

I am a nuvi 750 user. I bought a gomadic battery box for my gps. But I find out that if the internal battery is low, it useless. it won't charge the internal battery. I have #73 tip. I use rechargeable NIMH battery. My gps is fully loaded. I set the brightness at 50%. My gps only run at 4~5 hours. It seems that the battery box is useless.

The post you replied to would tend to indicate that you have a defective box OR you are doing something wrong. They wouldn't sell too many if they were truly useless. Maybe you should call the company or the place you bought it ???

It might be that you need to hook it up first then turn the unit on so it starts on a external power supply. I think if you hook it up while it is on, it will only run off the nuvi battery. This may be why you are not getting the gomadic battery. Try that and if it still does not work give them a call.

I have tried hook it up then turn the unit on when the internal battery is low. It went OFF right away when internal battery is dead. The battery would not charge the internal battery. I also tried turn off the unit while the battery box is on. It says "Charging battery Slide the power key to turn on". But after a while I check the internal battery status, it stays even less.
I will try your way and use regular battery (not rechargeable one) to see if it works for me otherwise give them a call.

If your nuvi is almost dead, don't try to force the charge. It is better if you hook it up to a your USB cable for awhile. There was a thread somewhere on here that said if your nuvi is below a certain level, the only way it would recharge is with the USB cable to your computer.

Here is the thread. It is possible that your battery is too low to charge the unit from the gomadic.

http://www.poi-factory.com/node/1212

NIMH are 1.2v so 4 would be 4.8v total. Alkalines are 1.5v, so 4 would be 6v total. It just might be the voltage difference that is causing this. You might want to try with 4 alkalines just to see if this makes a difference.

Press and hold the battery icon for 6 or 7 seconds to enter diagnostics - it shows the battery charge state.

That #73 tip contains the resistor, that signals to the GPSr "I am a Garmin charger". Implicit in that, is the ability to supply upto 1A of charging current. From what I've seen, this amount of current is only drawn for a short while, when the battery is completely flat. Maybe the Gomadic unit can't actually supply 1A?

(This might be the same sort of scenario you get, with a standard USB plugged into a Nüvi 3xx cradle. Excessive current is drawn, the voltage drops and the Nüvi says "Battery charging complete"...and it isn't.)

I did some tests this morning using both rechargeable and Alkaline batteries in my Gomadic Charger.

Test 1 was attaching Gomadic Charger with the Nuvi750 showing 2 bars. I let Nuvi on and waited 30 minutes and then unplugged the Charger and the Nuvi750 then showed 3 bars.

Test 2 was with the Nuvi750 with only 1 bar and the BEEP TONE alerting "LOW BATTERY". I plugged in the Gomadic Charger with 4 slightly used rechargeable batteries and then turned off the Nuvi. It showed Battery Charging. I left the Nuvi off for 45 minutes then turned the Nuvi back on, and it read 3 bars.

Test 3 was like test 2 but using 4 new Alkaline battries. This time after 45 minutes I unplugged the charger from the Nuvi and turned the Nuvi back on and the went into the diagnostic mode. Then I plugged the charger back in. The Nuvi shows CHARGING at 1 amp. And the battery capacity % was increasing.

So my supposition is that the charger will work even with a LOW battery. Not sure about if you let the battery fully discharge where it shuts down the Nuvi.

The way I use the charger is to use it as an external power source. When I use it to track my Amtrak trips I have it plugged in all the time.

That way I get about 14 hours of life before I have to change out the batteries in the charger.

Works for me in my application.

Thanks for your tests. I fully charged the gps battery (connected with computer over night) and started to test the gomadic battery box this moring. This time I hook it up first and turn it on the gomadic battery box. It is running over 6 hrs and It is still on!! (I just checked at 12 PM)
I used slightly used NIMH battery, the brightness is 50%.
I want to know that in all of your tests, do you turn off the gps first and turn it on with gomadic?
Have you ever tried turning it on when the Gps is already navigating?

I think there are two affecting the gps power
1. AA battery (1.2V versus 1.5V)
2. Gps must be off and fully charged before turn on the battery box

The next test I will use alkaline battery to see if it can charge the internal power.

Question 1: No I do not turn off the Nuvi first.

Question 2: Yes, there is no difference whether it is navigating or not as whether the charger works or not. The only thing that I can see in the difference is that if it is navigating it will use a little more poewr, but not enough to make a big difference.

The biggest item is the brightness level, there is a big difference of power drain between 20% and 100%.

I try to keep it as low as I can, but bright enough to be visible without straining my eyes. I keep it between 30% and 40% during day time, and 10% to 20% night time. This will add a lot of time to the usage.

In fact, the voltage of an Alkaline cell soon drops below that of a NiMH one, even though it starts off higher.

I took some measurements...
http://hornbyp.googlepages.com/RechargeableBatteries.htm

This graph show 2 different capacity NiMH cells and 1 Alkaline cell, discharging into a 14ohm resistor.

Also, as noted in this thread: http://www.poi-factory.com/node/10701, I think the Gomadic unit must contain a DC-DC convertor. Otherwise, the o/p from 4 AA cells (of either type), soon drops below the minimum USB spec.

I got no more than 25 minutes runtime, using the straight-forward approach of 4 cells in a battery pack, with no additional circuitry.

After couple tries, I figure out something interesting. I press and hold the power bar for 10 seconds, and the Nuvi 750 goes to BIOS menu.

When I plug in the car charger, the power source becomes External and the volts is 13.50 which is higher enough to charge the internal battery.

I then plug in the Gomadic battery box in mini usb hub, the nuvi 750 shows power cable in the USB ID column and USB in power source column. But the volts only output 3.80 which is same as operated by its internal battery. So I notice that the battery capacity only boots up for 5~10% only and it won't increase any more. The volts is too low to recharge the internal battery compare to the car charger. (I use full recharged NIMH 1.2V battery and the brightness is 50%). There is one thing I want to say is that no matter how low is the brightness, the volts is the same as 3.8V.

When using the nuvi 750, I notice the power capacity jumps up and down (5%~15%). Therefore when using Gomadic battery box, I will suggest to have the internal battery at least 40%, don't wait until last minutes, otherwise the gps may just turn off suddently in short period of time and you cannot recharge it back.

So my conclusion is that gomadic battery for my nuvi 750 is like second internal battery. If power capacity of the internal battery is 100%, then you will get another 100% etc.

PS. I will try Alkaline to see if it produce more volts

You are correct the voltage will stay the same no matter what brightness is set, (AT that specific slot in time) until you run the battery down. The next time slot checked the voltage will be lower, and so forth...... Even if you can't see it on a voltage meter.

But the brightness does affect the current (AMPs).
The BRIGHTER, the more current you use and the faster the battery WILL run down.

E=IR If "I"(current)is high and "R"(resistance) stays the same for all intent and purposes, E(VOLTAGE) will be lower. So the HIGHER the CURRENT the faster VOLTAGE will drop.

That's sounds good.

That's supposed to be the voltage it's seeing from the Gomadic unit (but it sounds more like the internal battery voltage). Does it say "Battery Charging"?

I'm suspicious, that you've either got a different unit to 'H Hannah', or it's defective.

Here are some screenshots of my Nüvi 760, in various charging configurations:-

Running on battery

http://hornbyp.googlepages.com/OnBattery.jpg

Charging from 12V, via cradle

http://hornbyp.googlepages.com/12VCharger.jpg

Charging from Garmin USB charger (from Nüvi 3xx) - (This is what it should look like, with the Gomadic)

http://hornbyp.googlepages.com/USBCharger.jpg

Charging from a PC USB port

http://hornbyp.googlepages.com/USBCharging.jpg

Under what conditions would you need the external battery unit when you are connected via a usb hub ??
I'm guessing none.

If you have a powered usb device available, you don't need the battery box, right?
So what's the point of "testing" it that way if it won't ever be used like that?

Or is there something in the picture that I am missing ??

I feel strange too, because when I charge from the usb charger with Gomadic, the volts is only 3.8 and it says "Battery Charging". I think that's the reason the Gomadic unit can't recharge my 750.

I use four 2500 mAH 1.2v NiMH battery. Maybe it wont output enough volts. I will try alkaline battery to see maybe the Gomadic unit produce more volts.

My mistake. I mean usb port on nuvi 750.

Yes. The power consumption is increased if the brightness is higher. I set the brightness 50% and the internal battery is 100%, then I will run my 750 more than 10 hrs with the Gomadic charger.

I turn on 750 in the BIOS menu with alkaline battery in the Gomadic charger and the volts is 4.60 to 4.80 which is good enough to charge internal battery. So the rechargable battery really matters. (Only output 3.8 volts).

For how long though?

That is still suspiciously low - the sort of voltage you'd get from a simple 4 cell battery pack - for a little while, at least.

I have a feeling that the DC-DC convertor (that the Gomadic must contain), has gone POP

See also:
http://www.poi-factory.com/node/10701

I went to Canadian Tire today to buy Alkaline battery and I found "Noma Pocket Power Charger" on sale, 40% off.

I strongly recommand this unit. Thumbs up!! It has usb port which I can even connect with original Garmin usb cable

This is the web site

http://www.canadiantire.ca/browse/product_detail.jsp?PRODUCT...