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Unsure on cache batteries

Old 07-18-15, 02:08 AM
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Unsure on cache batteries

Forgive me for not understanding but could someone explain to me how a cache battery can increase the length of time you can keep things running? Or have I got this wrong? I have a dynamo hub with a USB output. I'm considering getting a cache battery but I have no idea what mAh to get. Could someone provide me with a bit of guidance. If possible I'd like to power my tablet which requires 2A.
Thanks in advance
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Old 07-18-15, 02:37 AM
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It's a matter of supply and demand.

Basically amperes (amps, A) is just a unit of electrical current, it's measuring the number of coulombs worth of electrons that pass through a circuit in a second. Multiply that by a unit of time, such as hours, and you've effectively got a count of the number of electrons that the battery is theoretically capable of pumping through your device before it's expended and needs recharging.

So if you ran your tablet from a battery for an hour, it would consume two amp-hours. Smaller batteries are often measured in terms of milli-amp-hours or mAh, milli being the SI prefix for one thousandth. So 2Ah = 2000mAh.

It's worth pointing out two facts here:
- The 2A is likely to be a peak rating and that depending on what your tablet is doing, its overall consumption could be less than this.
- The rating of the battery will be impacted by a lot of factors and is at best a theoretical rating. As it becomes depleted, its output voltage will drop, possibly to levels below what is needed to power the tablet, so you may not be able to use all the theoretical capacity.

As for your dynamo… its generating current by your movement down the road. If for part of your journey, you're slugging it up hill, it's possible that your generated output is something less than the 2A needed by the tablet, so your battery has to pick up the slack, it's a net drain on the battery. Suppose you were able to muster about 300mA, the battery has to supply the remaining 1.7A.

Then you get over your hill, suddenly the generator is pumping in 4A. Supply is now in surplus so you'll be keeping the tablet alive and pumping up to 2A back into the battery.
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Old 07-18-15, 02:50 AM
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Thanks redhatter. Great explanation. I'm still unsure why routing the output from the dynamo to the USB then to a battery pack then to a device such as a tablet seems to be the preferred method for charging devices whilst cycling.

There are many cache batteries out there ranging from 1000 mAh to 20000 mAh. Since this is just a capacity does it matter if I just go with the lower end of the spectrum? The battery will be charging and discharging at the same time.

I've just found out my tablet battery has a capacity 5710mAh. It requires a 2A power supply.

Thanks
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Old 07-18-15, 02:51 AM
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Ah...I've just seen the rest of your reply redhatter. That's great. Am I right in thinking just go somewhere near the bottom of the range of capacities?
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Old 07-18-15, 02:59 AM
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Well, basically the idea is that when the dynamo isn't able to produce the required power, the battery steps up to take its place.

Years ago I had a dynamo light set-up on my bike with a small battery pack that had 3 (alkaline disposable) AA cells in it. When I slowed down, the pack switched to the batteries and so I continued to have lights when stopped. Soon as my speed picked up, it'd switch back to running off the dynamo.

Effectively it was a crude cache battery, without the charging ability. Once the AA cells were flat, it was dynamo-only. I'd imagine modern cache batteries do not have this limitation.

So in your situation, the tablet would continue to run off the cache battery even while you were stopped, at least until the battery was depleted.

As for capacity, I mentioned that the 2A would be a peak rating, and would include the power needed for the device itself and the embedded battery charging circuitry. Assuming worst case usage though, the 1000mAh would keep the tablet going half an hour, the 20Ah (20000mAh) would keep it going 10 hours.

The battery will either be charging or discharging, it can't do both. Basically if the dynamo is generating enough power to run your tablet and other devices, then the surplus will charge your battery. If there isn't the power from the dynamo, then the battery supplies the short-fall.
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Old 07-18-15, 03:04 AM
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Perfect. Thanks redhatter. That's all very clear
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Old 07-18-15, 03:18 AM
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One thing that I just remembered… the battery capacities are often measured at the 10-hour rate.

So a 1000mAh battery gets that rating because it is said to be able to supply 100mA for 10 hours. That often means if you try to drain 1A our of it, it might make it to an hour, but in probability, will start to drop off after 40 minutes or so, but could possibly run a 50mA device for over a day.

Take for example these three:

2.2 Amp Hour 12 Volt Sealed Lead Acid Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2482 | Jaycar Electronics
7.2 Amp Hour 12V Sealed Lead Acid Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2486 | Jaycar Electronics
12V 9Ah SLA Battery | SLA/GEL Rechargeable | Storage/Batteries | Power Products Electrical | PRODUCTS | SB2487 | Jaycar Electronics

So your mileage will vary. The harder you work a battery, the less time it will last. So my 30 minute estimate for the 1000mA pack might be on the optimistic side, it might be closer to about 25 or 20 minutes. The 20Ah pack though, if they've done their testing and design right, I'd be pretty confident should get very close to the 10 hours I mentioned earlier.
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