Lithium Batteries and Charging Systems for Custom Motorcycles – What You Need to Know!

Lithium Batteries In Custom Motorcycles

One of the challenges of building a custom motorcycle – particularly a café racer, is to strip it down to its essentials and lighten it up visually. So when it comes to making the bike look lean and nimble, less is definitely more. What separates the average build and a fantastic functional show-piece is removing side covers, hiding electrical components and wires, and so on. But where, and how do you hide that whopping battery that is commonly situated directly underneath the seat?

Hiding the battery or disguising it so it blends into other components is a must.

The idea is relatively straightforward, the smaller the battery, the easier it is to hide. The smallest batteries available come courtesy of lithium battery technology. Lithium battery technology is new and with it we’ve found great advantages in terms of energy density. You can now pack the same amount of power and energy in something a third of the size or less. You can also typically mount them in any orientation making them incredibly easy to stash away in inconspicuous places.

Check out the photo below; on the left is the battery that came with our recent Honda CB750 project, and on the right is an Antigravity 8 cell lithium battery. One weighs 5.4kg (12lbs), the other 1.8kg (4lbs).

However, with great power comes great responsibility. Each custom bike will bring different considerations and challenges in terms of where and how to hide it, but also what the ignition and on-board electrics require. In addition, we need to think about safety and ensure that the charging system is up to scratch.

Get these aspects wrong and you face a number of issues (some more serious than others) such as:

1. Trouble starting / not starting.
2. Starting but running out of capacity.  
3. Starting but overcharging.
4. Mechanical damage (Vibrations and heat), Shorting to ground, tension on cables etc.

Conditions 1 and 2 won’t damage anything, but it means lots of frustration instead. In a worst case scenario conditions 3 and 4 can eventually result in battery explosion and fire. More commonly however, it can cause the damage of other electrical components and potentially the loss of hundreds or even thousands of dollars in the process.

You may wonder why this is the case. As mentioned earlier, lithium battery technology is relatively new and we’re still coming to grips with how these batteries operate in vehicles like motorcycles. We’re also still exploring their volatility and end-of-life mode, and how to prevent mishaps that can cause a lot damage or even injury.

In short, some of the vulnerabilities that lithium batteries face are:

1. They are sensitive to excessive vibrations (apparently vehicle and road vibrations are okay – but it’s worth considering).
2. They are sensitive to heat (Most batteries have a temperature range that they prefer to operate in, but keeping it in a relatively cool place prolongs the life of the battery. Storing it near an engine with latent heat is okay but lithium batteries that sit directly up against a hot engine block are likely degrade very quickly! This is a big no-no).
3. They are vulnerable to over-discharging. Repeated over-discharging is likely to significantly degrade the battery in relatively few cycles. The batteries prefer to stay charged and stay within a certain range.
4. They are vulnerable to overcharging. Charging the battery at above the specified voltage for more than a few minutes can be catastrophic. The battery can become dangerously hot and or leak and even explode. Numbers 3 and 4 are probably the most common way to damage your battery so it’s incredibly important to make sure the charging system is reliable and suitable for charging a lithium battery. If you have an old bike, chances are that the charging system is not up to scratch and even if it is, it may not be capable of supplying the suitable voltage to a lithium battery. ie; A charging system that can supply up to 15 volts DC may cause no issue to a lead acid battery whereas a lithium battery could be damaged at this voltage. You’ll probably need some new components if you’re looking at using a lithium battery).

**A common failure mode for these batteries is to experience thermal runaway, swell, and potentially rupture/explode. Swelling occurs due to the degradation and breakdown of the positive and negative separators within the battery. This breakdown results in the build-up of flammable gas. If you’re lucky, the battery will swell and you will have caught it in time. If you’re less lucky it will rupture uneventfully leaving a mess behind. If you’re even less lucky, it’ll rupture and a positive and negative layer will touch, short-circuiting and causing the flammable gas to ignite.

Picking a Suitable Battery

What do you need to consider with each and every build to ensure you have a suitably rated battery and charging system?

1. Battery Cold cranking amps (CCAs).
2. Battery Capacity in amp hours (Ah).
3. Battery maximum and minimum charge voltage (V max/min).
4. Battery maximum charge current (A – the higher the charging current the quicker it’ll charge but there is a limit the battery can take).
5. Charging system Max voltage range (usually managed by your regulator/rectifier).

Let’s explore how we would go about assessing whether a lithium battery of a specific size and weight would be suitable for our project (Current project at the time of writing is a Honda CB750 F). The primary factors we want to match are CCAs, capacity in Ah (although this doesn’t have to be close because you’re likely using LED lighting which requires less current draw), and whether or not the original charging system is within the charging limits of the battery.

The battery in question is the ever popular 801 Antigravity Lithium Battery.

Honda CB750 F Electrical Specifications	Original Equipment *Lead acid or AGM Battery * original reg/rec	New (required) equipment * Lithium battery *lithium compatible reg/rec
Battery CCAs	210	240
Battery capacity	~16 Ah	9 Ah
Battery voltage range	11.8 to 14.8 V	12.4 to 14.2 V
Battery max charge rate	12 A	8 A
Charging System Max Voltage	~14.5 V (original reg/rec)	13.8V to 14. 2V (rick’s 14-403 lithium compatible reg/rec)
Charging System Supply Current	~4.5 A	~4.5 A

As shown in the table above, the new battery is comparable to the old – albeit significantly smaller and lighter. It has a smaller a capacity, but we’re not too concerned about that because current the bike will draw is very low due to the installation of motogadget LED lighting, instruments and components. The only required change is the regulator/rectifier because the old supply voltage will exceed the lithium batteries maximum voltage range. Remember, these batteries are very sensitive to overcharging so although the values are close, we really don’t want to be exceeding the limits of the battery. For this we have fitted a lithium compatible regulator rectifier specifically made for early 80’s Honda CB750s. We offer them in our shop courtesy of Rick’s Motorsport Electrics, so if you’re doing something similar check it out [here].

The Charging Circuit – The Role of the Regulator / Rectifier

Once you’ve installed all items it’s worth double checking everything is working as it should. This means starting the bike and checking the battery voltage at different RPM Ranges with the use of a multimeter. In the case of the custom CB750 , if the new lithium compatible regulator rectifier is doing its job correctly it should be charging at a level not exceeding 14.2 V.  But let’s get a bit a technical and see how this actually works. We’ll use our custom wiring diagram for the CB750 F found in the build blog.

Our main focus is the charging circuit so we’ve taken an excerpt from the diagram and shown it below.

The regulator rectifier is a simple device that is the brains of the charging system. It performs two main functions; 1. Regulating voltage by turning the stator on and off, and 2. Rectifying the 3 phase power from the stator to a DC supply back to the battery so it gets charged.

It does this by reading the voltage on the black ignition wire [1]. If the voltage is below the set voltage (say 14.2 V for lithium compatible components) the white ground potential wire [2] will close the circuit allowing the stator to begin charging.

The stator then spins and creates 3 phase power that is rectified by diodes that make up the rectifier. These diodes (there are 6 of them) then translate this into a DC power supply to the battery via the red wire [3] which is connected directly to the battery positive via our starter solenoid.

Sounds simple enough right? The main culprits of a bad charging system are poor connections, a faulty rotor, or worn brushes. It is absolutely crucial that all connections in this circuit are good, brushes are checked and the Stator/rotor assembly doesn’t short-circuit.

The fundamental tests should be conducted:

1. Check the resistance between the rotor slip rings. For this bike we’re looking at values between 3.6 – 5.2 ohms. It’s a good idea to check this cold, and when it’s hot (but be careful not to burn yourself).
2. Check the resistance between any yellow wires [4] on the stator. We’re looking for 0.4-0.5 ohms.
3. Check the continuity between any yellow wire and ground. We’re looking for no continuity.
4. Check the brushes aren’t worn past the wear indicators and the spring has adequate pressure.
5. Check the continuity of all grounds and resistances of all cables between connectors. (In some cases the connector can be the problem so be sure the connection is good and use electrical grease if you need to in order ensure a good connection).

Be sure to double check your earth wires. If the green earth connection [5] is poor then the system will not charge.

Be sure to also check your reference voltage, on the black ignition wire [1]. This is likely to be a problem area particularly with old electrics and wiring where a voltage drop could be caused by a bad contact somewhere in the circuit, ie; ignition switch. It should read a voltage very close to that of the battery. The volts on this cable will generally be lower than the battery due to the resistance of the cable and components in the circuit (usually up to 0.5 V lower).

If your reference reads something considerably off like 13.0 V but your battery is fully charged at 14.2 V. The stator circuit will close and the rectifier will continue to charge your already charged battery until the reference wire reaches 14.2 V. By this stage your lithium battery could be at 15.4V and rapidly degrading internally.

If everything checks out then you’re good to go. However it’s also not a bad idea for each build to include a voltage reader. This will monitor the voltage of the battery and if your charging system fails, you’ll notice the drop or increase in voltage from normal values and it will prompt you that something is wrong. If you’re using a motogadget motoscope pro, we recommend you have the secondary display as a voltage display. Note that you can also set the voltage range and if it falls out of this range you will get a warning on the display – very handy!

The above verification procedure will work for most bikes of the same vintage, be it a Suzuki, Yamaha, Kawasaki etc. But always consult the workshop manual for actual figures.

What about European classics like BMW?

If you’re building something like a BMW K or R Series bike then similar procedures apply. Special mention goes to the BMW K bikes which have quite onerous requirements in terms of current draw because of all the unsophisticated relays, control units and sensors that the 80’s brought with it. For these bikes the biggest considerations are ensuring you have the Ah capacity for the fuel injection system. BMW recommend a battery as high as 25Ah in capacity which can be difficult to find in a lithium variant. If you fall significantly short of this requirement then you risk damaging the EFI module because of continuous low voltages. At the same time, you could find your bike undercharging at lower RPMs. This means you could set off for a ride, have the bike purr along gracefully to the next lunch spot (if you make it there), and then find yourself unable to start the bike up again because they require quite a kick in CAs to get going.

Also, be sure that you double check that charging control circuit works. Removing the original instrument cluster (also referred to as the lunchbox) has a charging globe that provides enough resistance to allow for the charging system to activate and charge your battery. A lunchbox adapter like a 5 Aces Tenet module or a Marulabs BEP 3.0 will fix this along with other neutral starting challenges that come with the build.

In saying all that, we’ve had batteries as low as 16Ah run smoothly on K bikes, and as long as the charging circuit works normally and all the lighting is LED, then the capacity demands aren’t as high.

R Series bikes are carburetted so that makes them similar to our example above. But you’ll still need a new regulator rectifier if you’re going with the lithium route. For these custom builds it’s common to hide things very close to the engine. Keep in mind that your battery shouldn’t be getting too hot, and the regulator rectifier has cooling fins for a reason. Our recommendation is that you find a cool spot for mounting these. Under the tank or behind a seat cowl seems to be the appropriate place for most. Be sure to always do some basic tests.

Where to from here?

If you’re looking at upgrading your battery to a lithium ion battery be sure that it’s compatible with your charging system. The chances are that the regulator / rectifier isn’t, so head on over to our store to find yourself one that is.

Alternatively, if you need more advice, check out our other blogs or email us by using one of our contact forms on the site and we’ll do our best to help.