Rear light power supply.

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Rear light power supply.

Postby matt2matt2002 » 9 Oct 2020, 7:07pm

I need a new rear light.
Looking at Cateye and they are either battery or USB
Never had USB before.
Pros and cons/ opinions please.

Yes, other brands available but question about power source.
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Re: Rear light power supply.

Postby Pebble » 9 Oct 2020, 7:18pm

The problem I have found with the USB chargable ( li-ion?) batteries is they give a reasonably good light until they suddenly die/switch off. This means you get little to no warning and could go quite a few miles with no lights and not know. (as I have done in the past)
My front are both Rechargeable li-ion, and I would obviously know if one of them went off.

Traditional batteries seem to get dimmer and dimmer so you have many hours of warning that you need to replace.

I now have three sets of flashers on the back all powered by traditional 1.5v batteries. For instance I have known for the past couple of rides that one set is quite a bit dimmer and need new batteries

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Re: Rear light power supply.

Postby andrew_s » 9 Oct 2020, 7:28pm

Pebble wrote:Traditional batteries seem to get dimmer and dimmer so you have many hours of warning that you need to replace.

It depends when you check them.
It's fine if you check at the end of a ride, but if you only check when you turn them on, you could be riding round with glow worm lighting for a lot of time.

That's because traditional batteries recover somewhat when given a rest, and will give a good light for a minute or two when first turned on, only to revert to a dim glow soon after.

I use dynamo lights - permanently bolted to the bike so I don't have to remember to bring them, take them off when parking, or charge them, and they are always at full power. I don't even have to make the decision about whether to turn them on or not, as there's a photocell that does that for me.
Last edited by andrew_s on 9 Oct 2020, 7:32pm, edited 1 time in total.

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Re: Rear light power supply.

Postby rogerzilla » 9 Oct 2020, 7:30pm

Most USB rechargeables don't have enough battery capacity for an all-night ride.

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Re: Rear light power supply.

Postby thelawnet » 10 Oct 2020, 4:46pm

matt2matt2002 wrote:I need a new rear light.
Looking at Cateye and they are either battery or USB
Never had USB before.
Pros and cons/ opinions please.

Yes, other brands available but question about power source.

Sources of power for rear lights are largely:

1. Lithium polymer (can be made flat) - will be built in or a battery pack
2. Lithium ion (usually cylindrical) - will be built in or a battery pack
4. CR2032 (watch battery) - a stupid idea generally due to low capacity

USB is not the source of power, it's the charging mechanism.

Their best rear light was the Volt 50. This used a removable li-ion pack switchable with the front lights. The battery was 2200 mAh (though they have larger sizes) ... 60RC-REAR/

The Cateye batteries now are:

Sync Kinetic 500 mAh
Rapid X3 900 mAh
Rapid X2/X2 Kinetic, probably 500 mAh
Rapid X - not sure but probably 320 mAh
Rapid Mini 320 mAh

It depends what you are doing with your light, but lumens are directly proportional to power consumption effectively, and flashing will cut power usage a lot.

So the Rapid Mini claims 3 hours constant 25 lumens, from a 0.32Ah = 1.18Wh battery. That is 0.4W for 25 lumens, or 62.5 lumens per Watt.

The Rapid X3 claims 150 lumens for 1 hour, from 3.33 Wh, which is 45 lumens per Watt.

Their brightest AAA battery light is the Reflex Auto, which has 30 hours constant.

Two AAA batteries give around 900 mAh at 3V (vs. 3.7V from a li-ion battery), the difference is that alkaline drains faster at high loads, but 30 hours at 900 mAh is equivalent to only 0.03A/0.09W, which is almost nothing. Assuming 75 lumens/Watt, you are looking at around 7 lumens from the AAA light.

So if you want a VERY bright light, then AAA just won't be an option.

This curve shows the difference between disposable alkaline batteries and rechargeable NiMh:


Lithium is similar to NiMh.

Hence if you use alkaline it will get steadily dimmer throughout its life, whereas rechargeables will tend to simply die at some point.

It is for this reason that it's a good idea if buying a rechargeable light to get one that has a multi-stage battery indicator. More expensive lights (the Cateye ones do not really fall into this category) have this. Personally I think all the Cateye rechargeable models are quite cheap and nasty and not a patch on their AAA models.

If wanting a lithium light I would steer clear of Cateye and buy a Lezyne, Lupine, Exposure or similar. The Lezyne have a multi-stage battery indicator (depending on the model). And the Rotlicht is obviously very high quality. ... tlicht-max

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Re: Rear light power supply.

Postby Valbrona » 10 Oct 2020, 4:56pm

You don't say where you are going to mount it.

On my saddle rails I use a Cateye 5 Rapid (not all cateye lights attach to their rail mount bracket), while on my rear carrier I use a B+M 4D Toplight. Both battery. Both lights I would recommend.

Faffing around charging REAR lights is stupid. Save that for front lights unless you run one off a dynamo.
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Re: Rear light power supply.

Postby [XAP]Bob » 13 Oct 2020, 11:24pm

Dynamo lights done tend to power down. No faff involved.

Even when I did my own battery lights a decade ago I wired them together, just the one battery pack, so I knew that the rear was on...
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Re: Rear light power supply.

Postby simonineaston » 14 Oct 2020, 9:48am

Can't see any major downside to the stalwart combo of AA, or AAA, cells & LEDs, for a rear lamp... long-lived, simple, reliable, inexpensive to run, bright enough, wide choice of prices & designs. Nuff said.
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