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I've researched quite a bit but could not find a definitive answer to this.

Background: My old bike (already sold) had a starting problem - after a few months it would barely spin the engine up and require a replacement (8Ah). Most likely a decompressor problem since the starter was already exchanged, the terminals were clean, there was no measurable voltage drop over the wires. The cause of this issue is not my question.

The bike would ALWAYS start without a hitch if a small 2Ah 3S LiPo RC battery - or a normal car battery - was used in parallel. Without this help, the voltage would drop to 6-8V while cranking.

My question is: would a LiFePo4 battery replacement provide higher cranking current over the original gel battery, over a period of several months/years? For reference, the standard was a Yuasa YTX9-BS, but all lead-acid batteries I've tried performed almost exactly the same (Yuasa, Varta, Polo and Louis re-branded, no-name).

I've heard that AGM is "the best" and that LiPo is "even better" but "much more expensive". However, a standard maintenance-free battery is 30-50€, a LiFePo4 is 40-90€, AGM is >150€. I haven't seen standard LiPo in this size for sale, not through the usual suspects.

Are there any charts comparing standard lead-acid with LiFePo4 and AGM? Or at least some long-term experiences?

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    Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Dec 2 at 1:40
  • Thank you @Pᴀᴜʟsᴛᴇʀ2 ! – brainwash Dec 3 at 18:11
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I've heard that AGM is "the best" and that LiPo is "even better" but "much more expensive". However, a standard maintenance-free battery is 30-50€, a LiFePo4 is 40-90€, AGM is >150€. I haven't seen standard LiPo in this size for sale, not through the usual suspects.

If by LiPo you mean LiFePO4, then yes, in general lithium ion batteries, when properly used, have a longer cycle and calendar life than lead-acid batteries. They are more tolerant of long-term deep discharge and have lower rate of self-discharge. The energy density is higher and costs are quickly falling.

When improperly used, lithium ion batteries are likely to cause a fire. LiFePO4 is not as dangerous as some of the other lithium ion battery types. But LiFePO4 does not have as good performance as e.g. NMC batteries used in Tesla Powerwall grid energy storage applications.

I would classify a LiFePO4 retrofit to a motorcycle having a charging system for lead-acid batteries as a risky business. If it starts a fire, it's your fault not the battery manufacturer's fault.

Overall, I'm a little surprised why car / motorcycle manufacturers have not yet switched to lithium ion batteries in the 12 volt application. The reason can't be bulb life, because all important bulbs apart from perhaps turn signal indicators already use LED lights. Yes, electric cars / motorcycles have propulsion batteries mainly Li-Ion, but the 12 volt batteries have not yet switched to lithium ion.

Are there any charts comparing standard lead-acid with LiFePo4 and AGM? Or at least some long-term experiences?

You might be interested in reading this: https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries -- the figures are pretty compelling.

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I switched my '94 Suzuki Katana to LiFePo4 battery a few years ago. It cost a fair amount to get a quality one (and I opted to add-on the special charger which I've never even had to use). Provides lots of cranking power in the cold even when sitting for months. It has worked out well for me, but the manufacturers to have specific charging requirements, so make sure your make/model of bike is supported (and not modified electrically in a way that could cause problems).

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Lithium batteries are generally the best for power density, cycle life, cold performance, etc. I have some LiFePO4 cells that can candle 25C, while lead acid is typically rated for 1-2C continuous and maybe 5C max/cranking. This means an 8AH can deliver 40A for a few seconds, but the same size LiFePO4 can deliver 200A continuous.

LiPo (RC batteries) may help you start but are not a safe drop-in replacement due to their particular voltage and charging requirements. A 3S LiPo pack has a nominal 11.1V, and will drop to around 7.5V when fully drained, and 12.6V when fully charged. A lead-acid battery is about 10.5V fully drained and 13.8V fully charged. The charging system in a bike or car will overcharge a LiPo to the point of catastrophic failure if you leave it connected after you jump start.

LiFePO4 or Lithium Iron Phosphate cells are much safer and when used in a 4S configuration can be a drop in replacement as the voltage range is very close to lead acid. They still need a BMS to keep the cells balanced, but there are many of these batteries on the market which can fit right in to the same mount.

  • Just to be clear, the RC LiPo batteries were only used to jump-start and never left connected for more than a few seconds. I'm just writing this so other people don't attempt to leave them connected. – brainwash Dec 3 at 0:09

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