DC-DC Charger Calculator: Protect Your Alternator
DC-DC Charger Calculator: Protect Your Alternator
Want to charge your house battery while driving? In the old days, we used a $20 relay. Today, with Lithium batteries, that is a recipe for disaster. Lithium accepts charge so fast it can overheat and destroy your vehicle’s alternator. This tool by ShockTrail calculates the safe charger size for your specific engine.
The Alternator Safety Tool
Input your Alternator Rating (Amps) to see the maximum size DC-DC charger you can safely install.
DC-DC Sizing Tool
Vehicle Logic by ShockTrail0 Amps
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ℹ️ Keeps 50%+ of alternator power reserved for vehicle electronics.
The Problem: Lithium is “Too Hungry”
A standard lead-acid starter battery naturally resists charging as it gets full. A LiFePO4 battery does not. It acts like a black hole for energy.
If you connect a depleted 200Ah Lithium bank to a 100A alternator using a simple relay, the battery might pull 120A. The alternator will run at 100% duty cycle at idle (when the cooling fan is slow), leading to catastrophic thermal failure.
Leading manufacturer Victron Energy explicitly warns against direct alternator charging for this reason.
The 50% Safety Rule
To keep your alternator cool and leave enough power for the vehicle’s own electronics (lights, radio, ECU), ShockTrail engineers recommend limiting auxiliary charging to 50% of the alternator’s rated output.
| Method | Cost | Safety Level |
|---|---|---|
| Battery Isolator (Relay) | $20 – $50 | Dangerous for Lithium. OK for Lead-Acid. |
| DC-DC Charger | $150 – $400 | 100% Safe (Limits Current). |
Smart Alternators (Euro 5/6)
Modern vehicles (Sprinter, Transit, Promaster) have “Smart Alternators” that lower voltage to save fuel. A traditional relay will not work on these vans because the voltage drops too low to charge a house battery. You must use a DC-DC charger with “Engine Shutdown Detection,” like the Renogy DCC Series.
Frequently Asked Questions (FAQ)
How do I find my alternator rating?
Look for a sticker on the alternator itself, check your vehicle manual, or Google “[Year] [Model] Alternator Amps”. Common sizes are 90A, 150A, or 220A.
Can I run two DC-DC chargers in parallel?
Yes, if your alternator is big enough. For example, if you have a massive 300A dual-alternator setup, you can run two 50A chargers for faster charging.
Relevant Search Tags
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