Alternator for Volvo 2003t in the j40

[Michael Kahn]

Has anyone replaced the alternator on the Volvo 2003t? We finished our engine rebuild after we had a lot of saltwater corrosion that ate several hard pipes, but unfortunately the alternator also needs to be replaced. Apparently we have a 125 amp amptech s125e installed with an internal regulator, although the boat only has 3 batteries, so I wonder if this amperage is overkill? Anyone replaced the alternator on this engine and have any recommendations?

Thanks,
Michael

[Ed Redmond]

Hi Folks,

We've had really good luck so far with our electromaax. It 

• is optimized for marine applications with high output at low RPM
• is a reasonably easy installation - fits inside the OEM mechanical
• can be purchased with a pulley conversion kit that allows a more robust belt drive given the higher torgue associate with higher output currents
• associated regulator can be setup to be compatible with a conversion to Lithium if you want to eventually go that route
• has WiFi stats accessible underway
• has regulator parameter settings via USB cable
• has quite good tech support and the owner is an entertaining Brit. He'll get you in fine fettle.

Here's some datasheets if you want to peruse them quickly.

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[Tom Keffer]

I've also had an ElectroMaax 160 amp alternator for about 12 years and have been very happy with it. You definitely want to get the serpentine belt with it.

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[Sam Foster]

Michael, 

I know what I’m about to say will run contrary to the way some (many?) look at this but as a mostly coastal sailor and believer in the KISS principal of engineering, I’ve taken a different approach with our electrical. We have a lowly 60A (near) stock alternator. Same mounts, same belts. Works great for what we need. Let me explain a bit. 

I’ve kept the electrical system as close to stock as possible (it has near zero leakage!) BUT: we have upgraded to four 90Ah 980CCA X2 deep cycle AGMs. Constant updates on primary cables and wiring (but still far from all new), all lights converted with LED bulbs in stock fixtures, an old fridge that’s draws 4-5a (so do the new ones) and all new B&G NAV, AIS, Cell booster, 2500w inverter for 115v etc.  All wires and connections have liberal use of dielectric grease. 

So as I’ve said, it’s been great for what we do. Live on a mooring so no dock charging. Sail a few times weekly where the engine is only used for 10-15min at a time. We cruise for few weeks a season. More than a few days of sailing without running the engine is rare.  We never feel we’re lacking in power or need to run the engine much more than the typical harbor transit, low wind motoring etc. 

 —the only things we’re still considering are a few deck mounted walkable 100w solar panels. Which could let us run the fridge more but we don’t need that most of the time—

So on the charging: 

1. Battery capacity and depth of discharge

• The batteries are 12 V, 90 Ah.

• At 25% discharge, which is a lot, that means about 22 Ah needs to be replaced in each batttery.

2. Alternator rating

• The alternator is rated at 60 A at 12 V.

• In practice, alternators rarely deliver their full rated current continuously — heat, rpm, regulator behavior, and battery acceptance all reduce it. A safe assumption is ~50–70% of rating on average during recharge. Let’s assume 40 A average after further reducing for running equipment (which is what I see on my gauge) actually goes into the battery.

3. Ideal recharge time

• Energy to replace = 45 Ah.

• At 60 A continuous (perfect case): 2hr 16min

4. Absorption phase reality

Battery (Lead-acid or AGM) charging isn’t linear and they can only absorb so much at a time. Most notably, as the battery approaches ~80% state of charge (which is much of the time in most cases), current acceptance drops and the alternator output tapers. The last 20% may take just as long as the first 80%.

• To reach 80% charged (≈72 Ah): ~45–70 minutes.

• To reach nearly full (≈100%): often 2.5–3 hours total, even with a 60 A alternator. SO, at this point alternator size becomes much less important for the longer part of the change cycle. 

Sam

Cahoots

J/4O #44

⸻

On Sep 8, 2025, at 6:00 PM, Tom Keffer <tke...@gmail.com> wrote:



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[Wayne Cassady]

One small add to Sam’s very good comments. We too have four AGMs with a selector for house and start. We now carry one of the compact car start 

battery packs on board and have the boat selector set to “all” so that the lazy starting battery is being used. We have yet to run the four bank all the way down and used the start pack, but it provides peace of mind in case it happens.

Sent from my iPhone

On Sep 8, 2025, at 10:41 PM, Sam Foster <jsfo...@gmail.com> wrote:

Michael, 

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[Sam Foster]

Wayne is spot on, we do the same.

I’ll also add that we too have a rebuilt 2003t and when we bought the boat quickly blew the alternator (dumb error of a new owner switching batteries while running with an old selector switch.). That said we bought a new Bosch alternator and I had the old one rebuilt and carry it as a spare (that came Into use this summer!) You may not want to need a spare but there are generator rebuilders all around that can, usually quickly, rebuild your old one for much less than the cost of new. 

Sam

Sent from my iPhone

On Sep 9, 2025, at 8:10 AM, Wayne Cassady <wayne.c...@gmail.com> wrote:

One small add to Sam’s very good comments. We too have four AGMs with a selector for house and start. We now carry one of the compact car start 

To view this discussion visit https://groups.google.com/d/msgid/j4x-owners-group/B089261E-BD10-47CE-A4FE-5DB045A5B375%40gmail.com.

[Bill Bowers]

We have the Electromaax 250A remote rectified alternator now directly charging 2 Victron 330AH LFP primary House batteries. A single 100 AH AGM for engine start and a third get home 100AGM bank. The latter two are maintained by DC/DC  chargers. I derate the alternator to 200A and control it with a Wakespeed regulator linked to the BMS. 

The Electromaax has been trouble free. 

Bill Bowers

SV ConverJence

J42 #3

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