Every DC-DC charger gets a score from 0 to 100% built from five factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Chargers are grouped by current, up to 30 A and 40 A and up, and charge current has its own scale for each group.
| Factor | Measures | Weight |
|---|---|---|
| Value | price per amp of charge current | 30% |
| Charge current | rated output, A | 20% |
| Efficiency | maker's peak efficiency | 10% |
| Features | solar input, smart alternator support, Bluetooth, isolation | 20% |
| Warranty | warranty length | 20% |
Where the numbers come from
Charge current, efficiency, solar input, supported batteries, warranty and the rest come from the maker’s product page, data sheet or manual, or the maker’s own copy on its Amazon listing. Prices are checked at Amazon and the maker’s own store, and the lowest is used.
The factors
Value is the current price divided by the rated charge current, on a log scale, so going from $8 to $4 per amp counts as much as going from $16 to $8. $16 per amp scores zero and $4 per amp full marks.
Charge current is the rated output: for chargers up to 30 A, 10 A scores zero and 30 A full marks; for 40 A and up, 40 A scores zero and 60 A full marks.
Efficiency is the maker’s peak conversion efficiency: 88% scores zero and 97% full marks. Less loss means more of the alternator’s output reaches the battery and less heat in the charger.
Features counts four things that change what a charger can do: a built-in MPPT solar input, stated support for smart (variable-voltage) alternators, built-in Bluetooth monitoring, and galvanic isolation between input and output. Each is a quarter of the factor. An add-on Bluetooth dongle doesn’t count.
Warranty is the length of the standard consumer warranty: 1 year scores zero and 5 years full marks. Longer terms that need registration are listed but not scored.
What isn’t scored
Solar array size and voltage limits, supported battery types, ingress rating and weight are listed but not scored. Independent tests are listed where they exist, but there are too few to score.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.