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Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery

Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery
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$1,899.00
$1,899.00 - $1,899.00
$1,899.00
$1,519.00
$1,519.00 - $1,519.00
$1,519.00
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5.4kWh · 48V · LiFePO4

Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery

The eFlex MAX delivers 5.4kWh from a 48V LiFePO4 pack rated for 8,000 cycles at 80% depth of discharge, sealed in a saltwater-resistant aluminum enclosure rated for indoor or outdoor installation.

It parallels up to 16 units for 86.4kWh in one system and communicates over CANBus/Modbus with Fortress Power's Envy and Solo inverter lines.

Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery
10-Year Warranty
Lowest Price Guarantee
Authorized Fortress Power Dealer
Free Shipping
0% APR Financing
Expert Support: (303) 876-7654

Why the eFlex MAX

8,000 Cycles at 80% Depth of Discharge

Tier 1 automotive-grade prismatic LiFePO4 cells rated for 8,000 cycles at 80% DoD, with up to 100% usable depth of discharge.

>98% Round-Trip Efficiency

Minimal energy lost between charge and discharge, and under 2W standby draw in idle mode.

IP65, Saltwater-Resistant Enclosure

A ¾" aluminum shell rated IP65 for indoor or outdoor installation without an additional protective housing — including coastal environments.

Stack Up to 16 Units (86.4kWh)

Parallel up to 16 modules for up to 86.4kWh in one system, with an optional busbar package for higher current loads as you scale.

CANBus, Modbus, and Remote Monitoring

Communicates over CANBus and Modbus, with OTA firmware updates and remote monitoring through Fortress Power's app ecosystem.

10-Year Warranty

Fortress Power backs the eFlex MAX with a 10-year limited warranty.

What's Included

What's included: Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery

What You Get

  • 1 × Fortress Power eFlex MAX 5.4kWh Battery (48V LiFePO4)
  • Documentation

Full Specifications

Model Fortress Power eFlex MAX 5.4kWh
Nominal Voltage 51.2V
Voltage Range 48–55V
Nominal Capacity 105Ah
Total Energy 5.4kWh (5.374kWh rated at 0.5C/50A)
Battery Chemistry Tier 1 Automotive Prismatic LiFePO4
Max Charge Current 100A (recommended continuous: under 55A)
Recommended Continuous Discharge 60A (3kW)
Peak Discharge 100A (5kW) for up to 60 minutes
Cycle Life 8,000 cycles at 80% DoD
Depth of Discharge Up to 100%
Self-Discharge Under 1% per month
Round Trip Efficiency >98% (at 0.5C)
Charging Temperature Range 32°F to 114°F (0°C to 45°C)
Discharging Temperature Range -4°F to 131°F (-20°C to 55°C)
Storage Temperature Range 20°F to 95°F (-6°C to 35°C)
Dimensions 17.5"L × 24.2"W × 7.2"H
Weight 108 lb
Ingress Protection IP65
Communication CANBus, Modbus, OTA firmware updates, remote monitoring
Max Parallel Up to 16 modules (86.4kWh)
Compatible Inverters Fortress Power Envy 8/10kW, Envy True 12kW, Solo 6.5K, Guardian-monitored systems
Warranty 10 years
Certifications UL1973 (UL9540/UL9540A also listed at the system level)

Authorized Fortress Power Dealer

We sell genuine Fortress Power products backed by Fortress Power's own warranty.

Lowest Price Guarantee

Find it cheaper from another authorized dealer? Call (303) 876-7654 before you order.

Real Support, Not a Ticket Queue

Sizing a system or checking inverter compatibility? Call (303) 876-7654 and talk to a person.

Fortress eFlex Max 5.4 kWh 51.2V eFlex Max Battery: Frequently Asked Questions

Is this compatible with the standard eFlex 5.4kWh battery?
No — Fortress Power explicitly states the eFlex MAX is not compatible with the standard eFlex 5.4kWh battery, so plan your system around one or the other rather than mixing them.
What inverters does this work with?
Fortress Power's Envy 8/10kW, Envy True 12kW, and Solo 6.5K inverters, along with Guardian monitoring. Confirm compatibility with your specific setup — call (303) 876-7654 if you're not sure.
How many can I connect together?
Up to 16 modules can be paralleled for up to 86.4kWh total, with an optional busbar package for the higher current loads that come with a larger stack.
Does the max charge/discharge current mean I can run it at 100A continuously?
Not quite — 100A is the maximum rating. Fortress Power's recommended continuous charge current is under 55A, and recommended continuous discharge is 60A, with 100A available as a discharge peak for up to 60 minutes rather than sustained use.
Can this go outside?
Yes — its IP65-rated, saltwater-resistant aluminum enclosure is built for both indoor and outdoor installation without needing an additional protective housing.
Can I install this myself, or do I need an electrician?
Fortress Power's equipment is designed for professional installation. We strongly recommend a licensed electrician or solar installer for any battery, inverter, or enclosure installation — call (303) 876-7654 if you need a referral in your area.
Do you offer financing or price matching?
Yes. We offer 0% APR financing options through Affirm at checkout, and our Lowest Price Guarantee applies — if you find this product cheaper at another authorized Fortress Power dealer, call us at (303) 876-7654 before you order and we'll take care of it.
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