House banks · Usable capacity

Lithium vs AGM for a wake boat house bank

Updated 2026-10-11WakeTender engineering

Four Group 31 AGM batteries carry about 400 Ah on the label, give up roughly 200 Ah before you start shortening their life, and weigh around 260 lb. Four 120 Ah lithium iron phosphate batteries carry 480 Ah, give up nearly all of it, and weigh about 120 lb. For a wake boat that runs a stereo, a fridge, ballast heaters and lighting at anchor, the practical difference is more than double the usable energy at less than half the weight, and the voltage stays up until the end instead of sagging as the bank drains.

Usable capacity is the number that matters

Battery labels give nominal capacity. What you can use is different for each chemistry. A lead-acid battery, AGM included, should not be taken below about 50 percent state of charge on a regular basis without cutting its cycle life sharply, so a 100 Ah AGM is realistically a 50 Ah battery. Lithium iron phosphate is routinely cycled to 80 or 90 percent depth of discharge, and many manufacturers rate full depth, so a 120 Ah lithium battery is a 100 to 120 Ah battery in use.

Four-battery house bank	Group 31 AGM	WakeTender 120
Label capacity	~400 Ah	480 Ah
Usable capacity	~200 Ah	~480 Ah
Weight	~260 lb	120 lb
Voltage at 50% drained	~12.2 V and falling	~13.1 V and flat
Cycle life at the usable depth	A few hundred cycles	Thousands
Recharge from 50%	Hours, tapering	Accepts full charger output until nearly full

What that means at anchor

A 2027 flagship wake boat ships with a fridge-freezer, a 2,000 W inverter, a vacuum-flush head, heated seats, four cameras, a stern thruster, six subwoofers and always-on telematics, and runs them on roughly the same lead-acid bank a boat had twenty years ago. A 12 V compressor fridge over a long day at anchor uses roughly 40 to 60 Ah. A stereo played hard uses more. On a 200 Ah usable bank those are most of what you have; on 480 Ah they are a fraction, with margin for the lights and the head and a thruster burst on the way in.

Voltage matters as much as capacity. A lead-acid bank's voltage falls steadily as it drains, so amplifiers clip earlier and a thruster feels weaker at the end of the day. Lithium holds close to 13 V until it is nearly empty.

Charging is where lithium needs a plan

A big lithium house bank will accept whatever a charger can give it until it is nearly full. That is good news on shore power and bad news for a stock marine alternator, which can run at full field for a long time trying to refill a half-empty 360 Ah bank and overheat. The right configuration keeps the engine's charging circuit for the start battery and charges the house bank through a DC-DC charger and shore power. That also respects engine manufacturers' requirement that accessory loads over 30 A live on the house side. WakeTender's whole-boat layout is drawn this way.

Where lithium is not the answer

At the start position, a lithium battery that opens its circuit under charge can damage an inboard alternator, and most inboard engine manufacturers specify lead-acid or AGM there. Unless the lithium battery is engineered to keep the load path connected and warn before it acts, keep the start battery as specified and put the lithium in the house bank. See can you use a lithium starting battery in a wake boat?

Cost per usable amp-hour

Lithium costs more per battery and less per usable amp-hour over its life. A premium 120 Ah marine lithium battery retails in the $1,100 to $1,300 range in 2026; a Group 31 AGM is roughly $200 to $350. Four AGMs at $300 deliver about 200 Ah usable for a few hundred cycles: around $6 per usable amp-hour, replaced every few seasons. Four lithium batteries at $1,200 deliver 480 Ah usable for thousands of cycles: around $10 per usable amp-hour, bought once. If you count the weight you can put back into ballast and the afternoons you do not spend idling the engine to keep the stereo up, the gap closes further.

Questions

Can I mix lithium and AGM batteries on the same boat?

Yes, if the banks are kept separate and each is charged to its own profile; ABYC E-13's 2025 revision explicitly permits mixed chemistries. A lithium house bank and a lead-acid start battery joined by an ACR or DC-DC charger is a common and safe layout. Builders prefer a single chemistry for simplicity, which is what WakeTender is designed to allow.

Do I need a new charger for a lithium house bank?

You need a charger with a lithium iron phosphate profile, roughly 14.2 to 14.6 V absorption and a low float. Many modern marine chargers have one. A DC-DC charger between the alternator and the house bank is strongly recommended.

How much weight does lithium save on a wake boat?

A Group 31 AGM weighs about 60 to 70 lb; a 120 Ah lithium iron phosphate battery weighs about 30 lb. Four of each is a difference of roughly 140 lb, which is ballast you can place deliberately instead of carrying in the battery compartment.

WakeTender 120

A 12.8 V, 120 Ah lithium battery engineered to be safe at the start position of an inboard wake boat, and to scale to a 480 Ah house bank. See the specification or talk to us.