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So I've recently purchased an ebike and started thinking about how I could go long distances without charging.

It occurred to me that whilst sealed lead acid batteries can't go through many cycles, for the odd long distance (~100 miles) some deep cycle ones could be perfect, especially if mounted within a trailer.

I have a 48v e-bike, so I'd need 4 SLAs in series. I'd probably only use this a few times a year so other than the discharge from using them they'ed always be topped off with charge.

Can anyone see any potential disadvantages of doing this? Perhaps SLA's are just too heavy and I wouldn't get the range I want?

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  • I'd also team these up with a 16Ah LifePO battery which I think I'd get about 15 miles out of (~30mph/1000w motor): electricbikereview.com/forum/threads/…
    – John Hunt
    Commented Oct 24, 2018 at 13:57
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    If you want to do 30mph without pedalling, surely a moped (possibly of the electric variety) is a more appropriate tool for the job?
    – Andy P
    Commented Oct 24, 2018 at 15:17
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    "Perhaps... I wouldn't get the rage I want?" That's my favourite typo in a long time.
    – David Richerby
    Commented Oct 24, 2018 at 17:35
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    Lithium have about 3.5 times the energy density of lead. You then need a stronger (hence heavier) trailer to carry that weight, so work on a factor of at least 4x. 5kg of lithium will amount to nearly 20kg lead. If your e-bike has removable battery, beg, steal, borrow or rent spare battery packs (talk to the retailer).
    – mattnz
    Commented Oct 24, 2018 at 20:35
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    Remember batteries are consumable with a fixed lifetime often measured in full charge cycles. You could buy a second battery, and periodically swap them weekly during normal use, doubling their effective lifespans. And you now own two batteries to take on your longer rides. Downside, ebike batteries are not cheap.
    – Criggie ♦
    Commented Oct 25, 2018 at 11:13

3 Answers 3

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Lead acid batteries are heavy. Too much extra mass, even in a trailer could to put strain on the motor and brakes. You'd have to figure out what the mass of batteries you would need, and see if it would exceed the bike's maximum load specification.

Also, is it possible to run an eBike from an external DC power source?

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  • As to your last point, a switch in the battery leads would do the job on the ones I've dealt with. A simple modification, assuming you get a big enough switch. The Li battery protection circuit is in the battery pack itself, and the motor controller just expects a voltage.
    – Chris H
    Commented Oct 24, 2018 at 18:47
  • That’s not quite true. The charger is usually a CV/CC source and is current limited. If you present a 52.8 volts (which is a fully charged pack of lead acids) to a depleted 44 volt pack without current limitation, you could have quite a disastrous situation.
    – RoboKaren
    Commented Oct 25, 2018 at 6:21
  • I believe you can run an ebike from an 'external' DC source seeing as that's what the battery is anyway.
    – John Hunt
    Commented Oct 25, 2018 at 10:46
  • A battery is not a regulated voltage source. That’s what the BMC will expect.
    – RoboKaren
    Commented Oct 25, 2018 at 22:52
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A typical 12V, 12Ah SLA weighs about 3.5kg each. Four of them would weigh 14kg. Thats a lot of weight to haul around. Add the weight of the trailer, say 7kg, that's 21kg.

How are planning connecting them to the bike? I believe you can connect your 4 in-series SLAs in parallel with your Li-ion, but they're some caveats. You will need to monitor the voltage of your Li-ion and SLAs to prevent it from dropping too low, especially the Li-ion. Damage to Li-ion is probable. Connecting 4 SLAs in parallel is problematic as even identical model batteries have different characteristics. I think the better and safer method would be to switch to the SLAs when the Li-ion has discharged.

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  • Li-ion batteries typically have computers built into the battery to prevent over discharge, plus if in parallel there is nothing forcing the discharge on the Li-ion.
    – Rider_X
    Commented Oct 24, 2018 at 19:36
  • It would just be a straight swap when the lithium battery is dead. Nothing clever needed.
    – John Hunt
    Commented Oct 25, 2018 at 10:42
  • I'd guess OP may want to carry other stuff on the trailer too, like water and maybe camping gear, so the weight keeps going up. When the trailer weight hits half the weight of rider+bike then it starts to get noticeable in the handling. When the trailer weight equals the rider/bike weight, its really strange to ride.
    – Criggie ♦
    Commented Oct 25, 2018 at 10:57
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More downsides to carting a lot of batteries about

1) Recharging them. You'll need a 12V car charger which will take from 4 hours to overnight to do one battery. I've used a 3 way adapter to rewire three SLAs into one parallel 12V battery instead of the 36V serial battery needed to ride, and a smaller car charger took over 24 hours to charge them all at once.

You won't be able to use the bike's Lithium charger to charge the SLA batteries, because the chemistry is wrong. So it means packing along two chargers if you intend doing an out-and-back overnight trip.

2) SLA batteries are reasonably robust now, but many still contain liquid acid. "technically speaking" you should have a diamond-shaped hazardous cargo sign displayed. There is a risk of cracking, or I managed to wear partially through the plastic casing on one battery, so there's always a risk of an acid spill/release.

3) Wiring - your ebike will expect to receive 48V DC at roughly 6-7 Amps current into the system, assuming a 300W motor. The SLA batteries on the back will have to have a strong but thick power cable to get to the battery fitment. Remember thin wires means loss, so you'll expect something at least as thick as cheaper jumpstart leads.
It will have to follow the bike's draw-bar for support, but still have enough flex to turn corners. Likely you want to disconnect the trailer at some point for charging, so a plug in the line will be good too.

Perhaps you're approaching the ebike the wrong way. The electric motor is an assistant to your inputs, and its not the other way around. For your long rides, you aim to use the assist judiciously and only when its needed like climbs, not all the time.

Should the battery run flat, well you still have your legs. Downside is your flat ebike is now just a really heavy normal bike.

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    This whole thing was a theoretical question - I was just toying with the idea of using my new ebike to do long distances in short spaces of time without the need to buy a moped or use the car. I'm fairly anti fossil fuel and love cycling in all its forms. It sounds like getting more Lithium batteries is the way to go.
    – John Hunt
    Commented Oct 26, 2018 at 10:02

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