Start with the number printed on the battery, not the vehicle

Output amperage across these seven units spans tenfold, from the OptiMate 4 Dual Program’s 1A constant current to 10A on both the NOCO GENIUS10 and the OptiMate Lithium 4s. That spread looks like the decision, and it isn’t. The figure that tells you whether a charger was scoped to your battery is the amp-hour window, and six of the seven publish one: 5-25Ah for the CTEK CT5 POWERSPORT, 14-80Ah for the Battery Tender Plus, 3-50Ah for the OptiMate 4 (TecMate’s FAQ cites 3-48Ah for maintenance), 3-120Ah for the OptiMate Lithium 4s, and one-sided ceilings of 120Ah for the GENIUS5 and 230Ah in GENIUS10 retailer listings.

Those windows overlap less than the product categories suggest. The CTEK and the Battery Tender Plus share only the band from 14Ah to 25Ah; a 10Ah battery sits inside CTEK’s range and below Deltran’s stated minimum, while a 40Ah battery is the reverse. The Ah rating is printed on the battery itself, which is the only place that settles it — “it’s for a motorcycle” does not, since the windows here run from 3Ah to a 230Ah ceiling and the category name does not tell you which one applies.

Matching a specific stored battery

For a small powersport battery, two units publish windows that contain it at both ends. The CTEK CT5 POWERSPORT covers 5-25Ah at 2.3A with charge voltages of 14.4/15.8/14.2V published, and the OptiMate 4 Dual Program covers 3-50Ah at 1A. If that battery is LiFePO4, the CT5’s dedicated Lithium program applies, or the OptiMate Lithium 4s covers 3-120Ah; the OptiMate 4 lists STD, AGM and GEL lead-acid only.

For a car battery coming out of service for the season, the Battery Tender Plus’s 14-80Ah window at 1.25A is the one here stated at both ends for that size. The GENIUS5 publishes 5A to a 120Ah ceiling and the widest chemistry list of the group, but no minimum, so it is documented for the large end and silent on the small.

For a large LiFePO4 bank, the OptiMate Lithium 4s reaches 120Ah with AmpMatic current TecMate states adjusts to battery size and condition.

The SP1297 is the awkward case. It publishes 3A, four chemistries including LiFePO4, a case size and cable reach — but no Ah range, which means there is no published figure to check a specific battery against.

The maintenance claim, and what makes it checkable

Two different kinds of evidence back the storage behavior in this group, and they are not interchangeable.

The first is a described sequence. Deltran names four ISM stages, so the transition into maintenance is a documented step rather than an implication. TecMate publishes nine charging steps across two programs for the OptiMate 4 and ten for the Lithium 4s, and says the OptiMate 4 tests the battery before and after charging. CTEK publishes three named programs rather than a step count: Normal for lead-acid including AGM, Lithium for LiFePO4, and Recond for stratified or deeply discharged batteries.

The second is a quantified idle figure. TecMate gives standby power under 0.5W for the Lithium 4s. That number describes what the unit draws while sitting connected, which is a narrower claim than a stage description but a harder one to argue with.

Neither kind is present for two units. NOCO publishes no stage count or float voltage for either GENIUS model, and the GENIUS10’s own back-current figure was not confirmed in anything retrieved. Schumacher names float mode monitoring and Smart Trickle Charge and describes maintaining batteries without overcharging, but publishes no float voltage behind the name. “Automatic” on a box is not a specification; 0.5W is.

Cold storage splits the group sharply

If the battery is going into a heated basement, this factor costs nothing to ignore. If it is going into a shed, a barn or a boat, it separates the seven into three tiers. The comparison table carries each published floor, and the FAQ below sets out the three tiers.

That last tier is worth naming carefully: an unpublished figure makes a unit unverified for a cold garage, not unsuitable for one. The difference matters because it is the buyer who carries the uncertainty, not the manufacturer.

Chemistry is a one-way door

Lead-acid and LiFePO4 take different charge-voltage curves, and the lists here run in both directions. Deltran lists flooded, AGM, gel and sealed maintenance-free lead-acid, with no lithium. TecMate lists STD, AGM and GEL for the OptiMate 4, with no lithium. Going the other way, TecMate states the Lithium 4s is built for LiFePO4 and is not compatible with lead-acid at all.

Four units list both families: the GENIUS5 (wet, gel, MF, CA, EFB, AGM, calcium and lithium — the widest list of the seven), the GENIUS10 (flooded, gel, AGM, maintenance-free and LiFePO4), the SP1297 (standard, AGM, gel and LiFePO4), and the CT5, whose lithium support is specifically LiFePO4 and no other lithium chemistry. If you have swapped a powersport battery to lithium, or expect to, the chemistry list narrows the field before amperage does.

Starting voltage, for the battery you find already flat

Storage sometimes means a battery you get to late. The relevant spec is the voltage the charger will engage at, and most of these publish one. TecMate states recovery from as low as 0.5V on both OptiMate units, with 2-step desulphation on the OptiMate 4 and a 3-step save-and-test process on the Lithium 4s, whose charging range is given as 0.5V-14.4V. CTEK publishes start voltages of 2.0V for lead-acid and 5.0V for LiFePO4 — the highest lithium threshold stated here. No start-voltage figure was retrieved for the Battery Tender Plus or the SP1297.

When the low-output unit is genuinely enough

For a battery that stays in the vehicle and only needs holding at full charge, 1A to 2.3A inside a published window is exactly what the OptiMate 4, Battery Tender Plus and CT5 are scoped for, and all three document the maintenance end better than either NOCO does. Output becomes the deciding factor only when the same unit has to recover a large battery from low: 10A on the GENIUS10 against 1.25A on the Battery Tender Plus is an eightfold difference in stated current for that job, and the GENIUS10 covers to 230Ah where Deltran stops at 80Ah.

The trade-off that catches people out runs the other way. The two best-documented maintenance units are also among the lowest-output ones, and the highest-output lead-acid unit is the one with no published stage count and an unconfirmed back-current figure. No unit in this group is simultaneously the highest-current and the best-documented for sitting idle.

Details that decide the install

Cable reach varies enough to matter if the outlet is not next to the vehicle. Deltran publishes 12 ft total — a 6 ft AC cord, a 4.5 ft DC output cord and a 1.5 ft accessory cable. Both TecMate units publish a 6 ft AC lead plus a 6 ft charge lead terminating in an SAE connector. Schumacher publishes a 6 ft power cord with 5.17 ft of cable reach.

Input voltage is worth a glance if the unit will travel. The two NOCOs accept 120-240 VAC and both TecMate units accept 100-240V, while CTEK specifies 110-120 VAC and Deltran and Schumacher specify 120V. NOCO also publishes maximum input power: 75W for the GENIUS5 and 150W for the GENIUS10.

Warranty terms spread wider than almost anything else here: 10 years from Deltran, 5 from CTEK for the original purchaser, 3 from NOCO on both GENIUS units, 3 from TecMate on both OptiMate units, and 1 from Schumacher. Across a storage season repeated for years, that is the published figure closest to a statement about longevity — and it is the manufacturer’s statement, not an outcome anyone has measured here.