Short answer: the number of 750mAh spares you need depends on how many hours of the day you cannot recharge, not on how many frames you shoot. Carry two for street work and half-day outings; for full-day travel estimate one per 1.5 days; for long standby and interval shooting add one more, because standby draw depends far more on settings than on capacity. Go below that number and you will start counting bars at the exact moment you want to press the shutter.
A 750mAh battery holds roughly seven tenths of what a Canon original LP-E17 (1040mAh) holds, which makes it better suited to segmented use than to carrying a whole session on its own. The point of a spare-battery plan is to cut the day into blocks that each end with a swap.
First, work out the real runtime of one 750mAh battery
Runtime is not the same as standby time. With the camera switched on and idle, the current draw is very low, and a 750mAh battery can sit in the body for a day or two. Once you start framing, focusing and bursting, the draw climbs sharply. The table below therefore estimates active shooting hours, not hours away from home.
| Scenario | One 750mAh battery roughly covers | Carry | Notes |
|---|---|---|---|
| Street work, half-day outing | About 2-3 hours of active shooting | 2 batteries | Main battery plus one spare, no rush at the end |
| Full-day travel | About half a day | 1 per 1.5 days, minimum 2 | Work it out against hotel or cafe charging |
| Still life, studio | About 3-4 hours | 2 batteries | Power available, charge one while using the other |
| Long standby, interval shooting | About 6-10 hours | 2-3 batteries | Heavily affected by screen, Wi-Fi and interval settings |
| Video and time-lapse | About 40-70 minutes | 3 or more | Constant-power draw; not a good main battery at this capacity |
The times above are order-of-magnitude estimates and will move with the camera body, the lens and the temperature. Full-frame bodies such as the EOS RP and R8 draw more than APS-C bodies such as the R50 and R100, and the same battery can last thirty percent longer in one than in the other.
Street work: start with two, sized in half days
Street shooting is stop-and-go: the camera stays on for a long time while the frame count stays low. This is the easiest use case to underestimate, because you think you shot two hundred frames when in fact the body sat idle in the bag for three hours. Two 750mAh batteries is a comfortable setup. One goes in the camera, one goes in a pocket, and the weight is barely noticeable.
If the day will run long without a break, make the 950mAh main-capacity battery your primary and keep the 750mAh as a fallback. The reason is not capacity but opportunity: street work rarely offers a clean moment to swap, and a swap usually means missing the frame.
Travel: count days and charging chances
The travel calculation is total days divided by 1.5, rounded up, minus the nights where you are confident of a recharge. On a five-day trip with a hotel room every night, two 750mAh batteries and a two-bay charger will do. On a three-day hike with no outlet in between, you need four or more, or you move up to the 1050mAh long-runtime battery.
One detail that gets overlooked is how many sockets the room has. A two-bay dock charger solves more problems than an extra battery. For charger options, see the charging articles in the LP-E17 battery overview; charging specs are identical across all three capacity tiers.
Long standby and interval shooting: capacity is not the only variable
With interval work, star trails and monitoring-style standby, the draw comes mainly from a lit screen, Wi-Fi transfer and the periodic wake-up of autofocus, not from shutter actuations. Dimming the screen, turning the radio off and shortening the auto power-off timer usually buys more than another battery.
- Set auto power-off to 30 seconds or one minute before you leave, not to off.
- Turn Wi-Fi and Bluetooth off, and switch them on only when you need to transfer.
- Disable unnecessary lens stabilization, especially on a tripod.
- In cold weather, keep spare batteries in an inside pocket and fit them only when needed.
- Label each battery and store full and used batteries in separate pouches so you are not testing them on location.
FAQ
Do spares lose capacity if left unused for a long time?
Lithium-ion batteries age faster when stored fully charged. Charge a spare to roughly half before putting it away, top it up every three or four months, and never let it sit completely flat.
Does a 750mAh spare need to match the main battery’s capacity?
No, batteries of the same spec can be mixed freely. It does make sense to put the larger battery in the main shooting block, because the swap tends to fall in the middle of a shoot, which is the worst moment to need a second swap.
Are two 750mAh batteries enough for a day of travel?
That depends on your charging chances. If you return to a hotel every night, two will do. If you go two days without a socket, two are unlikely to be enough, particularly on a full-frame body. Move up to 1050mAh, or add a third battery.
Is it safe to keep spares in a camera bag?
Two precautions. Keep the contacts away from keys and coins, ideally in the original box or a battery case. And keep the bag out of a hot car in summer, because prolonged heat affects cycle life more than a deep discharge does.