Budget dual-battery approach using a $40 Cole-Hersee 24213 continuous-duty solenoid triggered by an ignition-switched +12V source. Repairable, no electronics to fry.
Before there were fancy DC-DC chargers, every dual battery setup used a 200A continuous-duty solenoid (Cole-Hersee 24213 or White Rodgers 70-111226). Wire the two large posts between batteries with 2 AWG, the small post 'S' to a switched +12V source through a 5-15A fuse, and ground the case. When you turn the key on, the batteries are paralleled; when you turn it off, they isolate.
Downsides: it doesn't wait for the alternator to come up to voltage before combining, so if you cranked hard the aux helps drag the main down on next start. It also doesn't actively manage charge state. Solution: add a manual override switch in series so you can disconnect before cranking. Or wire the trigger to an oil-pressure switch so the solenoid only closes when the engine is running.
For under $100 in parts you get 80% of the function of a Blue Sea ACR. This is the bulletproof, field-repairable choice for hardcore wheelers.
| Part | Vendor | Est. price |
|---|---|---|
| Cole-Hersee 24213 continuous-duty solenoid 200A | Amazon / NAPA | ~$40 |
| 2 AWG cable + lugs | WindyNation | ~$50 |
| MIDI fuse holders + fuses (x2) | Blue Sea Systems | ~$30 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.