A second battery lets the fridge, lights and laptops run all night without touching the battery that starts the engine. This guide explains why the second battery needs its own charger, how to size both, and what else goes in the box.
Why a second battery at all
A starting battery is built to deliver a huge current for a few seconds and then be refilled at once by the alternator. Running a fridge from it overnight is the opposite job: a small current for many hours, down to a deep state of discharge. Do that a few dozen times and a starting battery is finished, and one morning the engine will not turn over.
A deep-cycle battery is built for exactly that slow, deep work. Fullriver’s DC series uses thicker plates and more lead than a standard AGM and is rated by Fullriver to 80% depth of discharge, so a 115 Ah battery gives about 92 Ah of usable energy every night and comes back for more. Keeping it separate from the starting battery means the engine always starts, whatever the fridge did overnight.
Why a DC-DC charger beats a relay
The old way to charge a second battery was a voltage-sensing relay (a split-charge relay or VSR) that simply connects the two batteries while the engine runs. It worked when alternators held a steady 14.4 V. Modern vehicles are different:
- Smart alternators on many vehicles built in the last ten years drop the voltage to 12.5–13.5 V once the starting battery is full, to save fuel. At that voltage an AGM house battery barely charges at all.
- Voltage drop along six or eight metres of cable to the canopy or trailer takes another few tenths of a volt away.
- AGM batteries want a profile, not just a voltage. Fullriver specify 14.7 V absorption and 13.7 V float at 25 °C, adjusted for temperature. A relay cannot do that.
A DC-DC charger takes whatever the alternator gives it, boosts it, and charges the house battery with a proper multi-stage AGM (or lithium) profile at full current. KISAE’s Abso DMT chargers do this at 30 A, 50 A or 100 A, isolate the two batteries automatically, and come with a temperature sensor that trims the voltage to the real temperature of the battery, which matters in a Thai canopy at 45 °C. We include the sensor free with every compatible KISAE charger.
Sizing the battery
Add up what you run between drives, in amp-hours per day at 12 V. Rough figures for Thailand’s heat:
| Laden | Typical draw |
|---|---|
| Compressor fridge, 35–45 L | about 30 Ah a day |
| Compressor fridge, 60 L and up | about 45 Ah a day |
| LED lights and phone charging | about 10 Ah a day |
| A 12 V fan through the night | about 10 Ah a day |
| Laptop, per hour of use | about 5 Ah |
Multiply the daily total by the number of nights you want to sit without driving, then divide by 0.8 for Fullriver’s 80% depth of discharge. A fridge, lights and two hours of laptop come to about 50 Ah a day; two nights parked needs 100 Ah usable, which at 80% depth of discharge means a battery of at least 125 Ah. The DC145-12 (116 Ah usable) is the comfortable choice, and the DC115-12 (92 Ah usable) the compact one if you drive most days. The calculator on our homepage does this arithmetic for you.
Sizing the charger
Fullriver recommend a bulk charging current of about 20% of the battery’s 20-hour capacity, so a 115 Ah battery wants about 23 A and a 210 Ah battery about 42 A. That makes the KISAE DMT1230 (30 A) the match for banks from about 85 to 145 Ah, the DMT1250 (50 A) for 160–260 Ah, and the DMT12100 (100 A, with solar input) for two batteries or a camper that lives off its bank. A bigger charger also means a shorter drive puts the night’s energy back: an hour at 50 A returns about 45 Ah.
Away for a week?
Size the battery for the nights you sit still, then let driving and the sun refill it as you travel. An hour on the road with a 30 A charger returns about 27 Ah, and 200 W of solar on the roof about 50 Ah a day in Thai sun, so a fridge, lights and phones (about 40–50 Ah a day) keep going for as long as the trip lasts.
What else goes in the box
- Cable and fuses. Fuse both ends of the run between the batteries, close to each battery, and size the cable for the charger’s current and the length of the run; each KISAE charger page carries the manufacturer’s cable and fuse table.
- A battery monitor such as the Merlin SmartGauge, so you can see the state of charge rather than guess it.
- A secure tray or box that holds the battery against corrugations. AGM is sealed and can be mounted on its side, which gives you more options in a canopy.
- Solar if you park for days at a time; see solar and alternator charging in one box.
We size it for you, free
Send us what you run, how many hours you drive on a typical day and whether you have solar, on LINE @drakeco oder WhatsApp. An engineer replies with the battery, charger, cable and fuse list, usually within one working day. Everything in it is stocked or sourced by us in Thailand and checked before dispatch.
สรุปเป็นภาษาไทย
แบตเตอรี่สตาร์ทออกแบบมาเพื่อจ่ายกระแสสูงไม่กี่วินาที ไม่ใช่เพื่อเลี้ยงตู้เย็นทั้งคืน แบตเตอรี่ลูกที่สองแบบดีพไซเคิล (Fullriver DC series รองรับการใช้ลึกถึง 80%) ทำงานนี้ได้โดยไม่กระทบการสตาร์ทเครื่องยนต์
รถรุ่นใหม่ใช้ไดชาร์จอัจฉริยะที่ลดแรงดันเหลือ 12.5–13.5 V เมื่อแบตสตาร์ทเต็ม รีเลย์แยกแบตจึงชาร์จแบต AGM ไม่เต็ม เครื่องชาร์จ DC-DC ของ KISAE จะยกแรงดันและชาร์จตามโปรไฟล์ AGM ที่ถูกต้อง (14.7 V / 13.7 V ตามสเปก Fullriver) พร้อมเซ็นเซอร์อุณหภูมิที่เราแถมฟรี
วิธีเลือกขนาด: รวมการใช้ไฟต่อวัน (ตู้เย็นประมาณ 30–45 Ah ไฟและมือถือประมาณ 10 Ah โน้ตบุ๊กประมาณ 5 Ah ต่อชั่วโมง) คูณจำนวนคืนที่จอด แล้วหารด้วย 0.8 เลือกเครื่องชาร์จประมาณ 20% ของความจุแบต: DMT1230 สำหรับแบต 85–145 Ah, DMT1250 สำหรับ 160–260 Ah, DMT12100 สำหรับสองลูกหรือใช้งานหนัก ส่งรายการอุปกรณ์ที่ใช้มาทาง LINE @drakeco วิศวกรของเราคำนวณให้ฟรี