reading USB voltage with Garmin 800
Ken Shoemaker
Garmin 800? I'm using 4 AA batteries to run the device on the bike and
I'd like to get some idea of the state of the external batteries.
Ken
Scott Allen
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Thanks
From: Ken Shoemaker <k...@ieee.org>
To: randon <ran...@googlegroups.com>
Sent: Tuesday, May 24, 2011 7:44 PM
Subject: [Randon] reading USB voltage with Garmin 800
Does anybody know whether (and how) you can read the USB voltage from a Garmin 800? I'm using 4 AA batteries to run the device on the bike and I'd like to get some idea of the state of the external batteries.
Ken
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Jacques Bilinski
I don't know. But I'm sure there are very compact external devices
that can display the state of charge of 4 AA cells. I would search in
the radio control modeling sites for something like this. I've seen
little devices that display state of charge using a coloured LED bar
graph which RC hobyists use to display the voltage of their reciever
batteries.
Jacques Bilinski
installed inline between the battery and the load and measures a
number of things including the total energy used. I got one thinking
I'd use it for bike lights as a 'gas guage' to let me know how much
energy is left in my batteries and enable me to better optimize the
use of hi and lo settings on my bike lights. As it turns out however
it won't be useful as it is not accurate enough at low power settings.
I'm using an IXON IQ speed which only use 1/2W at the low power
setting. The meter is optimised for electric motors used by RC
hobyists with power in the 100s of Watts.
Keith Hearn
Even reading the voltage coming from the batteries (before the regulator) can be pretty deceiving, since many rechargeable batteries will keep a fairly steady voltage until they are almost out of charge, then have a fairly sharp voltage drop when they run out. This is very different from alkaline batteries that will have a fairly linear voltage drop as power is consumed.
So the only way to really know how much power you have left is to have something that tracks how much power has been used and compare it to the amount the batteries started with. This is what the "Watts Up" meter seems to do, but according to Jaques it's made for higher current levels. I'm not familiar with it at all. One could certainly make something for lower power usage (every smart phone or laptop has such a circuit), but I don't know if anything is commercially available in a standalone device. Unfortunately, a search for "usb power meters" gives lots of results for meters that measure othr kinds of power (radio, microwave, laser) and connect to a computer via USB to transfer the data, so anything that measure power flowing through a USB connection is kinda hard to find.
Even if you know how much power has been used, you still need to know how much the batteries started with. The rated capacity should be reasonably close for new batteries from reputable suppliers. But not all 2400mAH batteries really hold than much power. It's cheaper to label a battery as 2400mAH than it is to make one that really holds that much, and manufacturers know that very few consumers will ever test the capacity of a better. The only way to know would be to test each battery pack from full charge to empty from time to time and keep them labeled so you know what to expect.
In other words, it's non-trivial to know how much power you have left.
Keith
Does anybody know whether (and how) you can read the USB voltage from a Garmin 800? I'm using 4 AA batteries to run the device on the bike and I'd like to get some idea of the state of the external batteries.
Ken
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Erik Nilsson
conditioning mode that keeps track of how much charge batteries will
take and give back. At the end of the conditioning cycle, it reports
this information.
From this I have found that good but not high-end batteries from
reputable manufacturers will have significantly different charging
capacities as they age. Reconditioning seems to make this difference
even wider, as "good" batteries get better, but "bad" batteries only
get somewhat better and "dead" batteries stay fairly close to dead.
In a battery pack such as a 4 AA USB charger or even the 2 AAA
batteries in a Superflash (I assume, without taking one apart to
verify), the batteries are in series, which means capacity is not much
better than the weakest battery, especially if you charge and
discharge the pack as a unit. Most battery chargers charge batteries
in pairs, so with such a charger you are always charging a group of
two batteries.
So it makes sense to cull weak batteries out of packs and combine
batteries of similar capacities and identical chemistries. My
experience is that the voltage a battery produces after being charged
on a good negative delta V charger is related to charge capacity, but
the differences are in hundredths of a volt, I'm not sure how
correlated the result is, and with a charger that charges 2 batteries
at a time, the voltage you get for the stronger battery will be
brought down by the lower capacity of the weaker battery. Chargers
that charge batteries individually also usually report total charge
and discharge, so that's the easiest way to do it.
Why do batteries age differently? They may have had different
capacities when new due to variation in manufacturing quality. Age may
widen those differences. I think mechanical shock may be a factor. I
have definitely killed NiMH batteries by dropping them. I think many
NiMH batteries may be quite sensitive to shock and vibration, possibly
even fairly modest shocks and vibrations.
Hamish Moffatt
> In general, a USB source should be 5v +/- .25v at all times. Any decent
> power supply that has a USB socket should use a boost or buck circuit to
> regulate the voltage so it has a 5v output until the batteries are too far
> gone to maintain it, then it should shut down. Anything that just hooks
> batteries up to the USB socket without voltage regulation could harm USB
> devices that expect a regulated 5v.
The USB spec says 5V +/- .25V for high power hubs, but low power hubs
are allowed to be down to 4.4V. It's very unlikely that low voltage would damage
the device - more likely it would simply not work, or perhaps run the
Garmin without recharging it.
Any converter in the battery pack must be less than 100% efficient, so
you are wasting power. If the Garmin will work with a low voltage out of
the battery pack then you are better off without a converter.
I use a cheap battery pack containing 4xAAs. I put NiMH rechargables in.
I wouldn't be comfortable with alkalines (ie >= 6V on the USB socket)
but I have no concerns with the NiMH.
It'd be interesting to know what the output of the official Garmin
external battery pack is. It may be designed to power the 800 without
recharging it.
Hamish
Kole Kantner
voltages on a fully charged internal battery at 50% backlight. The 705
draws 60mA at 5.0V. Turning off the backlight drops the current to
55mA, which is not a lot for the added light. As the voltage rises the
current stabilizes at a slightly lower 48mA to 50mA up to about 7.4V.
By 7.5V the current has risen to 70mA and I did not continue testing in
that direction. At the low end the current increased to 65mA at around
4.3V. At 4.25 volts the current dropped to 9mA and by 4.20V the current
was 2mA, although the 705 still reports that it is charging as the
internal battery begins dropping.
After these tests I am comfortable continuing to use a simple 4AA
battery pack to run the 705. A set of 2000mAh rechargeable batteries
should provide around 30 hours of use and that matches what I observed
on a recent 600k. I recently tested fresh "2700mAh" batteries and they
show about 2550mAh available after a few complete cycles. That capacity
should provide about 40 hours of use before the internal battery begins
discharging. Due to the current limiting below 4.2V a reasonably
matched set of cells should not drop much below 1.1V per cell. Dropping
much below 1.0V begins to reduce the chargeable capacity, although most
damage occurs when the cells are allowed to drop to zero or even worse
to go negative.
I am comfortable using NiMh rechargeable batteries that start at up to
1.45V (5.8V) but drop almost immediately to 5V and stay very close to
4.8V for most of the discharge cycle. However, I am not sure about
alkaline batteries that start at about 1.6V each for 6.4V total,
although they drop very quickly to 1.5V and soon after that to 1.4V
totaling a reasonable 5.6V. Lithium AA start at 1.8V for a total of
7.2V, that might or might not be safe long enough for the voltage to
drop closer to USB standard 5V.
As others have mentioned using regulated voltage sources introduce
conversion inefficiencies and often increase the current draw greatly
reducing the battery efficiency. Some AA 5V supplies run a Garmin 705
less than an hour on two alkaline AA batteries. The best converters can
have efficiencies greater than 90%, although 80% is more common on good
ones. Bad ones are generally worse than useless as they fail when one
tries to rely on them. Testing in less critical situations, like on the
bike at home is recommended. Rechargeable batteries tend to provide
much better service, although it is useful to be able to purchase
disposable batteries during a ride if needed.
Kole
Kole Kantner
useful in some situations, although timing the discharge on a known
capacity set of batteries is usually good enough and much easier to
implement.
http://www.advantagehobby.com/product.php?productid=72936&cat=623
Kole
On 5/25/11 6:02 PM, Hamish Moffatt wrote: