Foil Drive Battery Guide: MAX+, Fusion, MAX and Slim
Choosing the right Foil Drive battery starts with one thing: identify exactly which Foil Drive system you own.
Assist MAX, Assist MAX+, Fusion and Assist Slim do not all share batteries, and some batteries that look or sound similar operate on completely different platforms.
The September 2026 launch of MAX+ makes this slightly easier in one area because the new FM+ battery family links MAX+ and Fusion. But it does not make Gen 2 MAX batteries compatible with Gen 2.5, and it does not bring Slim into the same battery family.
There is also more to battery choice than simply buying the highest Wh figure. Battery weight affects how the board feels, capacity affects how much stored energy you carry, and the amount of time you actually use the motor has a huge influence on real-world session length.
This guide explains what fits what, where the FM+ adaptor is required, which batteries suit different riders and why bigger is not always better.
Browse the Foil Drive range at The SUP Company alongside this guide, or contact us if you want us to check a battery against your existing setup.
Foil Drive battery compatibility: the quick answer
| Battery family | Assist MAX Gen 2 | Assist MAX+ | Fusion | Assist Slim |
|---|---|---|---|---|
| MAX Gen 2 batteries | Yes | No | No | No |
| FM+ 650 | No | Yes – direct fit | Yes – with FM+ adaptor | No |
| Fusion 860 | No | No | Yes | No |
| Slim batteries | No | No | No | Yes – correct Slim battery and charger required |
The FM+ Battery Adaptor does not make FM+ batteries compatible with the original Assist MAX.
Its job is physical: it allows the shorter FM+ battery to locate correctly inside the longer Fusion system. It does not convert an older Gen 2 system to Gen 2.5.
Foil Drive battery families explained
The easiest way to understand the range is to separate it into three main families.
Gen 2.5: MAX+ and Fusion
MAX+ and Fusion use Foil Drive's newer 43.2V architecture.
The FM+ 650 battery works across both systems:
- MAX+: direct fit.
- Fusion: fits with the FM+ Battery Adaptor.
Fusion can additionally use the larger 864Wh Fusion 860 battery.
Gen 2 Assist MAX
The original Assist MAX is a separate 40V battery platform.
Its battery range includes options such as MAX Sport, MAX Sport 396 High Power, MAX High Power and MAX Travel. These remain useful batteries for original MAX owners but cannot be fitted to MAX+.
Gen 2 Assist Slim
Assist Slim has its own battery range and physical format.
There is an additional complication here: not every Slim battery even uses the same voltage. The Slim Performance battery uses the 40V charging platform, while Slim Endurance is a 28V system requiring its specified 28V charger.
Never assume a charger is compatible simply because two batteries both fit a product called Assist Slim.
Foil Drive FM+ 650: the main MAX+ battery
The Foil Drive FM+ 650 is one of the most important products introduced with the Gen 2.5 range.
| FM+ 650 specification | Details |
|---|---|
| Rated capacity | 648Wh |
| Nominal voltage | 43.2V |
| Capacity | 15Ah |
| Battery weight | Approx. 3.95kg |
| Battery dimensions | 460 x 125 x 55mm |
| MAX+ compatibility | Direct fit |
| Fusion compatibility | Yes, with FM+ Battery Adaptor |
| Original MAX compatibility | No |
| Approx. stated charging time | 85 minutes with the correct 43V charger |
The naming is worth clarifying: FM+ 650 is the product name; 648Wh is the rated battery capacity.
What is different about the FM+ battery technology?
The new battery does more than store more energy.
Foil Drive's Gen 2.5 battery technology includes a more advanced battery-management system which communicates information including State of Charge and battery health through compatible Foil Drive equipment and the Foil Drive App.
Foil Drive also states that Gen 2.5 batteries are designed to maintain full power delivery until approximately 10% State of Charge before the normal reduction in performance becomes noticeable.
The battery and nose section are moulded into a single unit, rather than using the older separate battery and nose-cone arrangement. This is designed to improve sealing, durability and water flow around the system.
For storage, Foil Drive recommends following its current battery-care guidance and storing the FM+ battery at the specified State of Charge rather than simply leaving it fully charged for long periods.
Using the FM+ 650 with MAX+
The FM+ 650 fits the Foil Drive Assist MAX+ directly.
With Foil Drive's stated integrated High Power motor and three-blade configuration, MAX+ has a quoted minimum installed weight of approximately 6.71kg.
The complete installed arrangement is around 540mm long.
For a new MAX+ buyer, the FM+ 650 is therefore the natural launch battery and provides substantially more stored energy than the lighter batteries available for the original MAX.
That extra energy comes with extra weight, though. Riders coming from a lightweight original-MAX surf setup should not assume MAX+ plus FM+ 650 will automatically feel lighter simply because it is newer.
Using the FM+ 650 with Fusion
Fusion can use exactly the same FM+ 650 battery, but it requires the FM+ Battery Adaptor to locate the shorter battery securely in the longer Fusion housing.
This gives Fusion owners a genuinely useful choice.
You can use the smaller FM+ 650 for sessions where maximum capacity is unnecessary, then move to the much larger Fusion 860 for days when runtime is the priority.
With the FM+ 650 installed, Foil Drive quotes a minimum Fusion installed weight of approximately 7.13kg.
That is only around 420g more than MAX+ using the same battery, although the Fusion installation is longer at approximately 658mm.
Foil Drive's September 2026 Fusion specification includes the FM+ adaptor in the current base-kit contents. Existing Fusion owners should still check what is actually in their kit before ordering another, because packages supplied at different times can vary.
Foil Drive Fusion 860: maximum Gen 2.5 battery capacity
The Fusion 860 is the large-capacity option for Foil Drive Fusion.
It does not fit MAX+.
| Fusion 860 specification | Details |
|---|---|
| Rated capacity | 864Wh |
| Nominal voltage | 43.2V |
| Capacity | 20Ah |
| Battery weight | Approx. 4.9kg |
| Battery length | Approx. 575mm |
| Compatible system | Fusion |
| Approx. stated charge time | 120 minutes with the correct 43V charger |
The difference between the FM+ 650 and Fusion 860 is significant.
The 860 stores around 33% more energy, but weighs approximately 950g more.
That makes the choice relatively straightforward.
Choose the FM+ 650 in Fusion if: you want to reduce installed weight and do not need maximum capacity.
Choose the Fusion 860 if: longer powered sessions, repeated starts, heavier-rider use or e-foil-style riding justify carrying the additional battery.
FM+ 650 vs Fusion 860
| FM+ 650 | Fusion 860 | |
|---|---|---|
| Capacity | 648Wh | 864Wh |
| Battery weight | Approx. 3.95kg | Approx. 4.9kg |
| MAX+ | Yes | No |
| Fusion | Yes, with adaptor | Yes |
| Approx. charge time | 85 mins | 120 mins |
| Best fit | Balanced capacity and weight | Maximum Fusion capacity and powered-session potential |
How long do the FM+ batteries last?
Battery runtime is one of the most commonly misunderstood parts of Foil Drive.
A 648Wh battery does not mean every rider gets the same number of minutes on the water.
Foil Drive publishes indicative tests based around an 80kg rider, 800cm² front wing and 40-litre board.
| Manufacturer test | FM+ 650 | Fusion 860 |
|---|---|---|
| E-foil – intermediate | Approx. 31 mins | Approx. 41 mins |
| E-foil – advanced | Approx. 52 mins | Approx. 65 mins |
| Surf – intermediate | Approx. 40–50 mins | Approx. 50–80 mins |
| Surf – advanced | Approx. 55–70 mins | Approx. 80–110 mins |
These are comparison figures rather than guaranteed runtimes.
An experienced surf foiler may use the motor for only a few seconds at a time and spend much of the session riding with the motor off.
A rider learning flatwater starts may use large amounts of power repeatedly.
Rider weight, foil size, board drag, current, wind, motor height, propeller choice, riding style and the number of touchdowns all affect consumption.
Time on the water and time using the motor are not the same thing.
Original Assist MAX batteries
The arrival of MAX+ has not made original Assist MAX batteries obsolete.
If your Foil Drive Assist MAX Gen 2 is working well, choosing another battery within the correct MAX family may make considerably more sense than replacing the entire system.
The current MAX battery family includes several different approaches.
| Original MAX battery | Rated capacity | Best suited to |
|---|---|---|
| MAX Sport | 332.64Wh | Minimum weight and responsive surf-focused setups |
| MAX Sport 396 High Power | 396Wh | Low weight with more capacity and High Power capability |
| MAX Power | 498.69Wh | Older balanced-capacity option; now discontinued by Foil Drive |
| MAX High Power | 594Wh | Maximum current Gen 2 MAX battery capacity and performance |
| MAX Travel | Travel-specific modular battery | Assist MAX owners travelling by air, subject to airline approval |
All of these belong to the original Assist MAX platform. They do not fit MAX+ or Fusion.
MAX Sport: when minimum weight matters
The MAX Sport battery is the lighter option for riders who use Foil Drive mainly as an assist rather than a continuously powered system.
It makes particular sense for surf, pumping and downwind riders who may use a short burst for take-off and then switch the motor off.
Carrying extra energy that you rarely use does not necessarily improve the session. In this style of riding, reducing weight can make pumping, turning and general foil feel more important than maximising Wh.
MAX Sport 396 High Power
The MAX Sport High Power 396 battery is an interesting option for existing MAX owners.
It is rated at 396Wh and weighs approximately 1.85kg, creating a middle ground between the original lightweight Sport battery and Foil Drive's much larger MAX High Power option.
For a rider whose existing MAX works well but who wants a little more energy without moving onto the heavier end of the battery range, this may be a more sensible upgrade than changing the whole system to MAX+.
MAX High Power: maximum capacity for the original MAX
The MAX High Power battery is rated at 594Wh and is designed for the original Assist MAX.
Foil Drive quotes a battery weight of approximately 2.8kg.
When paired with the appropriate High Power motor setup, it also supports the higher Boost capability available within the Gen 2 MAX system.
This makes it particularly relevant for existing owners who need more capacity and performance but do not specifically need the new Gen 2.5 battery ecosystem.
Assist Slim batteries
Assist Slim remains a completely separate battery format from both MAX and Gen 2.5.
It is designed for riders prioritising minimal weight and compactness, so its battery capacities are much smaller than MAX+, Fusion and the larger MAX batteries.
Slim Performance
The Foil Drive Slim Performance Battery is rated at approximately 166Wh and weighs around 1kg.
It uses the 40V Slim platform and is aimed at riders wanting a lightweight, punchy assist for starts rather than large amounts of continuous powered runtime.
Slim Endurance
The Slim Endurance battery increases capacity to approximately 241Wh, but there is an important electrical difference.
Slim Endurance is a 28V battery and requires the specified 28V charger.
Do not connect it to a 40V charger simply because another Assist Slim battery uses 40V.
Slim Travel
The Slim Travel battery uses modular battery sections designed to make airline travel more practical.
It remains an Assist Slim battery. It is not compatible with MAX, MAX+, Fusion or the FM+ battery system.
What about Foil Drive travel batteries?
Foil Drive produces separate modular travel batteries for original MAX and Assist Slim.
The MAX Travel Battery is for Assist MAX.
The Slim Travel Battery is for Assist Slim.
Neither is an FM+ battery and neither works with MAX+ or Fusion.
The individual battery sections are designed around air-travel restrictions, but that does not guarantee an airline will accept them.
Airlines can have their own rules, prior-approval processes and limits on spare lithium batteries. The rules can also depend on route and operator.
Before travelling:
- Identify the exact battery model.
- Keep the manufacturer's battery specification documentation.
- Check the Wh rating of each individual travel module.
- Contact the operating airline for current approval requirements.
- Obtain written approval where the airline requires it.
Do not rely solely on the words “Travel Battery” when arriving at check-in.
What about the FM+ 430?
Foil Drive's September 2026 Gen 2.5 launch material refers to a smaller FM+ 430 battery planned for early 2027.
This is potentially important because a smaller Gen 2.5 battery could change the weight-versus-runtime balance of MAX+ considerably.
However, until Foil Drive releases the final specification and availability information, we would not build a current purchase around assumptions about its final capacity, weight or package configuration.
If minimum MAX+ weight is important to you, contact us for the latest information as the release approaches.
Battery capacity, weight and power are not the same thing
Watt-hours tell you stored energy
A battery's Wh figure tells you how much energy it can store.
It does not tell you the maximum motor thrust, the acceleration of the board or exactly how long your session will last.
Battery weight changes how the setup feels
A large battery can be brilliant if you need its energy.
If you spend most of the session with the motor switched off, though, you are still carrying that battery while pumping, carving and carrying the board back up the beach.
That is why experienced surf riders sometimes deliberately choose a smaller battery even when a larger option is available.
The motor determines how quickly you spend the energy
Repeated flatwater starts, long periods motoring upwind and e-foil-style use consume energy much faster than a few seconds of assistance to catch a wave.
Two riders can both say they had a “one-hour Foil Drive session” while one may have used the motor for five minutes and the other for forty.
Which Foil Drive battery should you choose?
| Your situation | Where we would start |
|---|---|
| Buying a new MAX+ | FM+ 650 at launch; check exact package contents before ordering |
| Fusion but want lower installed weight | FM+ 650 plus the correct adaptor |
| Fusion and want maximum capacity | Fusion 860 |
| Original MAX used mainly for surf assistance | Compare MAX Sport and MAX Sport 396 |
| Original MAX needing maximum current battery capacity | MAX High Power 594Wh |
| Original MAX already working well | Stay within the compatible MAX family rather than changing system automatically |
| Slim rider prioritising low weight | Slim Performance |
| Slim rider wanting additional capacity | Compare Slim Endurance, including its different voltage and charger |
| Flying with Assist MAX | MAX Travel battery plus airline approval |
| Flying with Assist Slim | Slim Travel battery plus airline approval |
Charging: match the charger to the battery
This is an area where guessing is particularly unhelpful.
- MAX+ / FM+ / Fusion: use the specified 43V charging equipment.
- Original MAX: use the compatible 40V Foil Drive charger.
- Slim Performance: uses compatible 40V charging equipment.
- Slim Endurance: requires its specified 28V charger.
Original MAX and MAX+ chargers are not interchangeable.
Likewise, never transfer charging instructions from one Slim battery to another without checking the exact battery specification.
Battery care and storage
Foil Drive batteries are a major part of the value of the complete system, so good battery care matters.
Follow the current Foil Drive manual supplied for your exact battery for charging, storage, inspection and long-term care.
For Gen 2.5 batteries, Foil Drive recommends storage at a partial State of Charge rather than leaving the battery permanently full.
Inspect the battery, connectors and seals as part of your normal setup routine. If a battery has been damaged, dropped, swollen, is behaving unusually or shows signs of water ingress, stop using it and seek appropriate technical advice.
This is also important when buying a used Foil Drive. The condition and age of the battery can be just as important as the condition of the motor and electronics.
Do you need the biggest Foil Drive battery?
Not necessarily.
Battery choice is one of the areas where buying the highest specification can actually make the setup less suitable.
Choose more battery when you use more battery.
If you are a heavier rider making repeated starts, motoring for long periods or using Fusion as an e-foil alternative, additional capacity can be extremely useful.
If you mainly need a short burst to get onto foil and then spend the session surfing or pumping with the motor switched off, saving weight may improve the overall experience more than adding unused Wh.
The right choice is the battery that gives you enough usable energy for your normal sessions without unnecessarily compromising the way the board feels.
Foil Drive battery FAQs
Is the FM+ 650 actually a 650Wh battery?
FM+ 650 is the product name. Foil Drive gives the battery a rated capacity of 648Wh.
Can I use FM+ 650 with MAX+?
Yes. The FM+ 650 fits MAX+ directly.
Does MAX+ need the FM+ Battery Adaptor?
No. The adaptor is used when fitting the shorter FM+ battery into Fusion.
Can the FM+ Battery Adaptor make FM+ 650 work with my original MAX?
No. Original MAX and MAX+ use different electrical platforms. The adaptor only deals with physical fit inside Fusion.
Can Fusion use FM+ 650?
Yes. Fusion can use the FM+ 650 with the correct FM+ Battery Adaptor.
Can MAX+ use the Fusion 860 battery?
No. The 864Wh Fusion 860 is specific to Fusion.
Which is lighter, FM+ 650 or Fusion 860?
The FM+ 650 weighs approximately 3.95kg while the Fusion 860 weighs approximately 4.9kg, a difference of around 950g.
Can I use my old MAX battery in MAX+?
No. Original Assist MAX batteries are Gen 2 40V batteries and are not compatible with the Gen 2.5 MAX+ system.
What is the biggest battery for the original Assist MAX?
Foil Drive's current MAX High Power battery is rated at 594Wh and is designed specifically for the original Assist MAX.
Is the MAX Sport 396 compatible with MAX+?
No. The MAX Sport High Power 396 is an original Assist MAX battery. It does not fit MAX+.
Are all Slim batteries 40V?
No. Slim Performance uses the 40V platform, while Slim Endurance is a 28V battery requiring its specified 28V charger. Always check the exact battery rather than assuming all Slim charging equipment is interchangeable.
Can I use a MAX Travel battery in MAX+?
No. MAX Travel is designed for the original Assist MAX. MAX+ uses the newer FM+ battery platform.
Can I take a Foil Drive battery on a plane?
That depends on the exact battery and airline. Foil Drive's modular MAX and Slim Travel batteries are specifically designed to make airline transport more practical, but airline approval can still be required. Check the current rules with the operating airline before travelling.
Will a larger battery always give me a longer session?
It gives you more stored energy, but real session duration depends on rider weight, board and foil efficiency, conditions and how much you use the motor. If the motor is switched off for most of the session, carrying the largest battery may offer relatively little benefit.
Is the FM+ 430 available now?
Foil Drive's September 2026 launch information indicates an early-2027 release. Until final specifications and availability are confirmed, treat it as a future option rather than a current buying choice.
Check your Foil Drive battery before ordering
Battery compatibility is one part of Foil Drive where it pays to check before pressing buy.
Send us the name of your Foil Drive system, a photograph of your current battery label and what you want to achieve. If you are changing battery because you need more runtime, less weight, more performance or easier travel, tell us that too.
We can help identify whether you need an FM+ 650, Fusion 860, original MAX battery, Slim battery or simply a different approach to the setup you already own.
Ask The SUP Company to check your Foil Drive battery choice, browse the Foil Drive range or visit The SUP Co x Lymington for specialist foil-assist advice.
Finance options are available on qualifying orders, which can be useful when investing in a complete Foil Drive setup. Battery and system availability can vary, so if timing matters please ask us to confirm current stock or lead time.